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  • richardmitnick 10:54 am on July 27, 2019 Permalink | Reply
    Tags: "Ask Ethan: Can We Really Get A Universe From Nothing?", , , , , Because dark energy is a property of space itself when the Universe expands the dark energy density must remain constant., , Cosmological constant, , , , Galaxies that are gravitationally bound will merge together into groups and clusters while the unbound groups and clusters will accelerate away from one another., , Heisenberg uncertainty principle, Negative gravity?, ,   

    From Ethan Siegel: “Ask Ethan: Can We Really Get A Universe From Nothing?” 

    From Ethan Siegel
    July 27, 2019

    1
    Our entire cosmic history is theoretically well-understood in terms of the frameworks and rules that govern it. It’s only by observationally confirming and revealing various stages in our Universe’s past that must have occurred, like when the first stars and galaxies formed, and how the Universe expanded over time, that we can truly come to understand what makes up our Universe and how it expands and gravitates in a quantitative fashion. The relic signatures imprinted on our Universe from an inflationary state before the hot Big Bang give us a unique way to test our cosmic history, subject to the same fundamental limitations that all frameworks possess. (NICOLE RAGER FULLER / NATIONAL SCIENCE FOUNDATION)

    And does it require the idea of ‘negative gravity’ in order to work?

    The biggest question that we’re even capable of asking, with our present knowledge and understanding of the Universe, is where did everything we can observe come from? If it came from some sort of pre-existing state, we’ll want to know exactly what that state was like and how our Universe came from it. If it emerged out of nothingness, we’d want to know how we went from nothing to the entire Universe, and what if anything caused it. At least, that’s what our Patreon supporter Charles Buchanan wants to know, asking:

    “One concept bothers me. Perhaps you can help. I see it in used many places, but never really explained. “A universe from Nothing” and the concept of negative gravity. As I learned my Newtonian physics, you could put the zero point of the gravitational potential anywhere, only differences mattered. However Newtonian physics never deals with situations where matter is created… Can you help solidify this for me, preferably on [a] conceptual level, maybe with a little calculation detail?”

    Gravitation might seem like a straightforward force, but an incredible number of aspects are anything but intuitive. Let’s take a deeper look.

    2
    Countless scientific tests of Einstein’s general theory of relativity have been performed, subjecting the idea to some of the most stringent constraints ever obtained by humanity. Einstein’s first solution was for the weak-field limit around a single mass, like the Sun; he applied these results to our Solar System with dramatic success. We can view this orbit as Earth (or any planet) being in free-fall around the Sun, traveling in a straight-line path in its own frame of reference. All masses and all sources of energy contribute to the curvature of spacetime. (LIGO SCIENTIFIC COLLABORATION / T. PYLE / CALTECH / MIT)

    MIT /Caltech Advanced aLigo



    VIRGO Gravitational Wave interferometer, near Pisa, Italy


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    LSC LIGO Scientific Collaboration


    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

    Gravity is talking. Lisa will listen. Dialogos of Eide

    ESA/eLISA the future of gravitational wave research

    Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018


    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    If you have two point masses located some distance apart in your Universe, they’ll experience an attractive force that compels them to gravitate towards one another. But this attractive force that you perceive, in the context of relativity, comes with two caveats.

    The first caveat is simple and straightforward: these two masses will experience an acceleration towards one another, but whether they wind up moving closer to one another or not is entirely dependent on how the space between them evolves. Unlike in Newtonian gravity, where space is a fixed quantity and only the masses within that space can evolve, everything is changeable in General Relativity. Not only does matter and energy move and accelerate due to gravitation, but the very fabric of space itself can expand, contract, or otherwise flow. All masses still move through space, but space itself is no longer stationary.

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    The ‘raisin bread’ model of the expanding Universe, where relative distances increase as the space (dough) expands. The farther away any two raisin are from one another, the greater the observed redshift will be by time the light is received. The redshift-distance relation predicted by the expanding Universe is borne out in observations, and has been consistent with what’s been known going all the way back to the 1920s. (NASA / WMAP SCIENCE TEAM)

    NASA/WMAP 2001 to 2010

    The second caveat is that the two masses you’re considering, even if you’re extremely careful about accounting for what’s in your Universe, are most likely not the only forms of energy around. There are bound to be other masses in the form of normal matter, dark matter, and neutrinos. There’s the presence of radiation, from both electromagnetic and gravitational waves. There’s even dark energy: a type of energy inherent to the fabric of space itself.

    Now, here’s a scenario that might exemplify where your intuition leads you astray: what happens if these masses, for the volume they occupy, have less total energy than the average energy density of the surrounding space?

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    The gravitational attraction (blue) of overdense regions and the relative repulsion (red) of the underdense regions, as they act on the Milky Way. Even though gravity is always attractive, there is an average amount of attraction throughout the Universe, and regions with lower energy densities than that will experience (and cause) an effective repulsion with respect to the average. (YEHUDA HOFFMAN, DANIEL POMARÈDE, R. BRENT TULLY, AND HÉLÈNE COURTOIS, NATURE ASTRONOMY 1, 0036 (2017))

    You can imagine three different scenarios:

    1.The first mass has a below-average energy density while the second has an above-average value.
    2.The first mass has an above-average energy density while the second has a below-average value.
    3.Both the first and second masses have a below-average energy density compared to the rest of space.

    In the first two scenarios, the above-average mass will begin growing as it pulls on the matter/energy all around it, while the below-average mass will start shrinking, as it’s less able to hold onto its own mass in the face of its surroundings. These two masses will effectively repel one another; even though gravitation is always attractive, the intervening matter is preferentially attracted to the heavier-than-average mass. This causes the lower-mass object to act like it’s both repelling and being repelled by the heavier-mass object, the same way a balloon held underwater will still be attracted to Earth’s center, but will be forced away from it owing to the (buoyant) effects of the water.

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    The Earth’s crust is thinnest over the ocean and thickest over mountains and plateaus, as the principle of buoyancy dictates and as gravitational experiments confirm. Just as a balloon submerged in water will accelerate away from the center of the Earth, a region with below-average energy density will accelerate away from an overdense region, as average-density regions will be more preferentially attracted to the overdense region than the underdense region will. (USGS)
    6

    So what’s going to happen if you have two regions of space with below-average densities, surrounded by regions of just average density? They’ll both shrink, giving up their remaining matter to the denser regions around them. But as far as motions go, they’ll accelerate towards one another, with exactly the same magnitude they’d accelerate at if they were both overdense regions that exceeded the average density by equivalent amounts.

    You might be wondering why it’s important to think about these concerns when talking about a Universe from nothing. After all, if your Universe is full of matter and energy, it’s pretty hard to understand how that’s relevant to making sense of the concept of something coming from nothing. But just as our intuition can lead us astray when thinking about matter and energy on the spacetime playing field of General Relativity, it’s a comparable situation when we think about nothingness.

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    A representation of flat, empty space with no matter, energy or curvature of any type. With the exception of small quantum fluctuations, space in an inflationary Universe becomes incredibly flat like this, except in a 3D grid rather than a 2D sheet. Space is stretched flat, and particles are rapidly driven away. (AMBER STUVER / LIVING LIGO)

    You very likely think about nothingness as a philosopher would: the complete absence of everything. Zero matter, zero energy, an absolutely zero value for all the quantum fields in the Universe, etc. You think of space that’s completely flat, with nothing around to cause its curvature anywhere.

    If you think this way, you’re not alone: there are many different ways to conceive of “nothing.” You might even be tempted to take away space, time, and the laws of physics themselves, too. The problem, if you start doing that, is that you lose your ability to predict anything at all. The type of nothingness you’re thinking about, in this context, is what we call unphysical.

    If we want to think about nothing in a physical sense, you have to keep certain things. You need spacetime and the laws of physics, for example; you cannot have a Universe without them.

    8
    A visualization of QCD illustrates how particle/antiparticle pairs pop out of the quantum vacuum for very small amounts of time as a consequence of Heisenberg uncertainty.

    The quantum vacuum is interesting because it demands that empty space itself isn’t so empty, but is filled with all the particles, antiparticles and fields in various states that are demanded by the quantum field theory that describes our Universe. Put this all together, and you find that empty space has a zero-point energy that’s actually greater than zero. (DEREK B. LEINWEBER)

    But here’s the kicker: if you have spacetime and the laws of physics, then by definition you have quantum fields permeating the Universe everywhere you go. You have a fundamental “jitter” to the energy inherent to space, due to the quantum nature of the Universe. (And the Heisenberg uncertainty principle, which is unavoidable.)

    Put these ingredients together — because you can’t have a physically sensible “nothing” without them — and you’ll find that space itself doesn’t have zero energy inherent to it, but energy with a finite, non-zero value. Just as there’s a finite zero-point energy (that’s greater than zero) for an electron bound to an atom, the same is true for space itself. Empty space, even with zero curvature, even devoid of particles and external fields, still has a finite energy density to it.

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    The four possible fates of the Universe with only matter, radiation, curvature and a cosmological constant allowed. The top three possibilities are for a Universe whose fate is determined by the balance of matter/radiation with spatial curvature alone; the bottom one includes dark energy. Only the bottom “fate” aligns with the evidence. (E. SIEGEL / BEYOND THE GALAXY)

    From the perspective of quantum field theory, this is conceptualized as the zero-point energy of the quantum vacuum: the lowest-energy state of empty space. In the framework of General Relativity, however, it appears in a different sense: as the value of a cosmological constant, which itself is the energy of empty space, independent of curvature or any other form of energy density.

    Although we do not know how to calculate the value of this energy density from first principles, we can calculate the effects it has on the expanding Universe. As your Universe expands, every form of energy that exists within it contributes to not only how your Universe expands, but how that expansion rate changes over time. From multiple independent lines of evidence — including the Universe’s large-scale structure, the cosmic microwave background, and distant supernovae — we have been able to determine how much energy is inherent to space itself.

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    Constraints on dark energy from three independent sources: supernovae, the CMB (cosmic microwave background) and BAO (which is a wiggly feature seen in the correlations of large-scale structure). Note that even without supernovae, we’d need dark energy for certain, and also that there are uncertainties and degeneracies between the amount of dark matter and dark energy that we’d need to accurately describe our Universe. (SUPERNOVA COSMOLOGY PROJECT, AMANULLAH, ET AL., AP.J. (2010))

    This form of energy is what we presently call dark energy, and it’s responsible for the observed accelerated expansion of the Universe. Although it’s been a part of our conceptions of reality for more than two decades now, we don’t fully understand its true nature. All we can say is that when we measure the expansion rate of the Universe, our observations are consistent with dark energy being a cosmological constant with a specific magnitude, and not with any of the alternatives that evolve significantly over cosmic time.

    Because dark energy causes distant galaxies to appear to recede from one another more and more quickly as time goes on — since the space between those galaxies is expanding — it’s often called negative gravity. This is not only highly informal, but incorrect. Gravity is only positive, never negative. But even positive gravity, as we saw earlier, can have effects that look very much like negative repulsion.

    Dark Energy Survey


    Dark Energy Camera [DECam], built at FNAL


    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

    Timeline of the Inflationary Universe WMAP

    The Dark Energy Survey (DES) is an international, collaborative effort to map hundreds of millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that will reveal the nature of the mysterious dark energy that is accelerating the expansion of our Universe. DES began searching the Southern skies on August 31, 2013.

    According to Einstein’s theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet, in 1998, two teams of astronomers studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up. To explain cosmic acceleration, cosmologists are faced with two possibilities: either 70% of the universe exists in an exotic form, now called dark energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.

    DES is designed to probe the origin of the accelerating universe and help uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 400 scientists from over 25 institutions in the United States, Spain, the United Kingdom, Brazil, Germany, Switzerland, and Australia are working on the project. The collaboration built and is using an extremely sensitive 570-Megapixel digital camera, DECam, mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, high in the Chilean Andes, to carry out the project.

    Over six years (2013-2019), the DES collaboration used 758 nights of observation to carry out a deep, wide-area survey to record information from 300 million galaxies that are billions of light-years from Earth. The survey imaged 5000 square degrees of the southern sky in five optical filters to obtain detailed information about each galaxy. A fraction of the survey time is used to observe smaller patches of sky roughly once a week to discover and study thousands of supernovae and other astrophysical transients.

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    How energy density changes over time in a Universe dominated by matter (top), radiation (middle), and a cosmological constant (bottom). Note that dark energy doesn’t change in density as the Universe expands, which is why it comes to dominate the Universe at late times. (E. SIEGEL)

    If there were greater amounts of dark energy present within our spatially flat Universe, the expansion rate would be greater. But this is true for all forms of energy in a spatially flat Universe: dark energy is no exception. The only different between dark energy and the more commonly encountered forms of energy, like matter and radiation, is that as the Universe expands, the densities of matter and radiation decrease.

    But because dark energy is a property of space itself, when the Universe expands, the dark energy density must remain constant. As time goes on, galaxies that are gravitationally bound will merge together into groups and clusters, while the unbound groups and clusters will accelerate away from one another. That’s the ultimate fate of the Universe if dark energy is real.

    Laniakea supercluster. From Nature The Laniakea supercluster of galaxies R. Brent Tully, Hélène Courtois, Yehuda Hoffman & Daniel Pomarède at http://www.nature.com/nature/journal/v513/n7516/full/nature13674.html. Milky Way is the red dot.

    So why do we say we have a Universe that came from nothing? Because the value of dark energy may have been much higher in the distant past: before the hot Big Bang. A Universe with a very large amount of dark energy in it will behave identically to a Universe undergoing cosmic inflation. In order for inflation to end, that energy has to get converted into matter and radiation. The evidence strongly points to that happening some 13.8 billion years ago.

    When it did, though, a small amount of dark energy remained behind. Why? Because the zero-point energy of the quantum fields in our Universe isn’t zero, but a finite, greater-than-zero value. Our intuition may not be reliable when we consider the physical concepts of nothing and negative/positive gravity, but that’s why we have science. When we do it right, we wind up with physical theories that accurately describe the Universe we measure and observe.

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 10:28 am on January 29, 2019 Permalink | Reply
    Tags: , , , Cosmological constant, , , Quasars are brilliant enough to be seen from a universe less than a billion years old making them prime targets for reaching earlier epochs, , , Two decades ago astronomers discovered that the universe was not only expanding but accelerating in its expansion, Type Ia supernovae have long been the brightest of standard candles, What Quasar Cosmology Can Teach Us About Dark Energy   

    From Sky & Telescope: “What Quasar Cosmology Can Teach Us About Dark Energy” 

    SKY&Telescope bloc

    From Sky & Telescope

    January 28, 2019
    Monica Young

    Astronomers have found a way to turn quasars into standard candles, with potentially far-reaching implications for the nature of mysterious dark energy.

    Standard Candles to measure age and distance of the universe NASA

    National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — Funded BICEP2 Program; modifications by E. Siegel.

    Two decades ago astronomers discovered that the universe was not only expanding but accelerating in its expansion. They dubbed the cause of this acceleration dark energy, but what that actually is remains as ineffable now as it was then.

    The weird repulsive force has left its fingerprints on the earliest photons we can see, the ones emitted as part of the cosmic microwave background (CMB), when the infant universe was only 370,000 years old. Yet dark energy only began to dominate expansion as the universe entered middle age, after 9 billion years or so.

    Now, Guido Risaliti (University of Florence and INAF-Astrophysical Observatory of Arcetri, Italy) and Elisabeta Lusso (Durham University, UK) are using quasars to probe the cosmology of our universe’s relatively unexplored adolescence. The results, appearing in the January 28th Nature Astronomy, promise to reveal dark energy’s true nature.

    The leading explanation for dark energy has long been the cosmological constant, also known as vacuum energy. This energy inherent to empty space arises from quantum theory, which says that even when space appears empty of particles, it’s actually filled with quantum fields. These fields exert a negative pressure that counteracts the attractive force of gravity. However, calculations of vacuum energy overpredict the measured dark energy density by an astounding 120 orders of magnitude (that’s a 1 followed by 120 zeroes!). That the cosmological constant remains the favorite theory speaks to how little we understand dark energy — and how difficult the measurements involved are.

    Studying the universe at any age starts with gauging cosmological distance — the farther we look, the further back in time we see­­ ­— but we can’t just roll out a tape measure to the stars. Enter standard candles, objects for which we can measure an intrinsic luminosity. By comparing how bright a standard candle appears to be with how bright it really is, we can determine its distance without knowing anything about cosmology.

    Type Ia supernovae have long been the brightest of standard candles. Observations of these detonating white dwarfs led to the Nobel-winning discovery of accelerating expansion announced back in 1998. The supernovae extended our reach to when the universe was a third of its current age. That’s a pretty good tape measure! Nevertheless, it only probes the era when dark energy began to dominate the universe’s expansion. To see farther back, and probe the era when dark energy overtook matter, astronomers need something even more luminous.

    Quasars as Standard Candles

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    Understanding the physics of quasar accretion disks (blue-white) and X-ray-emitting coronae (yellow) can help astronomers use quasars as standard candles.
    NASA / CXC / M. Weiss.

    What’s more luminous than an exploding star? A gas-guzzling supermassive black hole would do the trick. After all, quasars are brilliant enough to be seen from a universe less than a billion years old, making them prime targets for reaching earlier epochs.

    Unfortunately, quasars also exhibit a bewildering variety of forms — astronomers have long thought they were anything but standard. Case in point: Astronomers have known for the past 30 years that more visibly luminous quasars emit relatively fewer X-rays, but there was too much variance from one quasar to another to pin down any one quasar’s intrinsic brightness.

    Risaliti and Lusso realized that this relation between the emission of X-rays and visible light must arise from the physics of quasar accretion disks. The disk itself emits visible light, while a hot, gaseous corona emits the X-rays. The two are intertwined by straightforward physics; it’s just that previously, contaminants had been mucking things up. So for this study, Risaliti and Lusso removed any sources where disk emission is obscured (by dust or gas) or contaminated (by emission from a fast-flowing black hole jet). Their careful selection results in a much tighter, more useful relation. Using data from the Sloan Digital Sky Survey and the XMM-Newton, Chandra, and Swift space telescopes, the duo then apply the relation to turn 1,600 quasars into standard candles.

    SDSS 2.5 meter Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft)

    ESA/XMM Newton

    NASA/Chandra X-ray Telescope

    NASA Neil Gehrels Swift Observatory

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    The history of the universe shows a crucial time when the expansion switched from decelerating to accelerating. But the future still hangs in the balance, depending on the behavior of dark energy. If dark energy increases, everything will be torn apart; if it changes direction, the cosmos could end in a big crunch.
    NASA / CXC / M.Weiss

    The quasars help Risaliti and Lusso fill in the gap along the cosmic timeline, looking back to an adolescent universe only a billion years old. From this data, the team finds that dark energy is actually increasing over cosmic time.

    The results appear to rule out the cosmological constant, which predicts a constant energy density. That’s a bit of a relief given that vacuum energy overpredicts the observations so badly. (Did I mention the 120 orders of magnitude?) Evolving dark energy may also help resolve an ongoing tension between measurements of the universe’s current expansion rate.

    Nevertheless, the results are unsettling from a philosophical standpoint: If dark energy density really does increase over time, then so does the repulsive force it exerts, potentially ending our universe in a Big Rip.

    Too Early To Tell

    Let’s not give up on the universe just yet, though. Phil Hopkins (Caltech), who wasn’t involved in the study, urges caution in interpreting its results. The relation that Lusso and Risaliti use to turn quasars into standard candles may itself evolve over time, making those quasars not so standard. For example, if quasars slow their gas-guzzling as mergers become less frequent, that might change the shape of the relation between the emission of X-rays and visible light. “[The relation] only needs to evolve a little bit to explain these observations,” he adds.

    That said, Hopkins agrees the results are interesting and worth following up with even bigger and better samples. The authors also note that other studies probing the adolescent universe are forthcoming. The bar is high these days for disproving the standard cosmological model, and only time and additional study will tell if this is the method that will do it.

    See the full article here .
    See also from Chandra here.

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

    Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

    Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

    “Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”

     
  • richardmitnick 7:59 am on September 27, 2017 Permalink | Reply
    Tags: , , , Cosmological constant, , , Dark energy may not exist, ,   

    From COSMOS: “Dark energy may not exist” 

    Cosmos Magazine bloc

    COSMOS Magazine

    27 September 2017
    Stuart Gary

    1
    A model of the universe that takes into account the irregular distribution of galaxies may make dark energy disappear. NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team and ESA

    The accelerating expansion of the universe due to a mysterious quantity called “dark energy” may not be real, according to research claiming it might simply be an artefact caused by the physical structure of the cosmos.

    The findings, reported in the Monthly Notices of the Royal Astronomical Society, claims the fit of Type Ia supernovae to a model universe with no dark energy appears to be slightly better than the fit using the standard dark energy model.

    The study’s lead author David Wiltshire, from the University of Canterbury in New Zealand, says existing dark energy models are based on a homogenous universe in which matter is evenly distributed.

    CMB per ESA/Planck

    ESA/Planck

    “The real universe has a far more complicated structure, comprising galaxies, galaxy clusters, and superclusters arranged in a cosmic web of giant sheets and filaments surrounding vast near-empty voids”, says Wiltshire.

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al

    Current models of the universe require dark energy to explain the observed acceleration in the rate at which the universe is expanding.

    Scientists base this conclusion on measurements of the distances to Type 1a supernovae in distant galaxies, which appear to be farther away than they would be if the universe’s expansion was not accelerating.

    Type 1a supernovae are powerful explosions bright enough to briefly outshine an entire galaxy. They’re caused by the thermonuclear destruction of a type of star known as a white dwarf – the stellar corpse of a Sun-like star.

    All Type 1a supernovae are thought to explode at around the same mass – a figure known in astrophysics as the Chandrasekhar limit – which equates to about 1.44 times the mass of the Sun.

    Because they all explode at about the same mass, they also explode with about the same level of luminosity.

    This allows astronomers to use them as standard candles to measure cosmic distances across the universe – in the same way you can determine how far away a row of street lights is along a road by how bright each one appears from where you’re standing.

    2
    Standard candles. https://www.extremetech.com

    On a galactic scale, gravity appears to be stronger than scientists can account for, using the normal matter of the universe, the material in the standard model of particle physics, which makes up all the stars, planets, buildings, and people.

    To explain their observations, scientists invented “dark matter”, a mysterious substance which seems to only interact gravitationally with normal matter.

    To explain science’s observations of how galaxies move, there must be about five times as much dark matter as normal matter.

    Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016

    It’s called dark because whatever it is, it cannot emit light. Scientists can only see its effects gravitationally on normal matter.

    On the even larger cosmic scales of an expanding universe, gravity appears to be weaker than expected in a universe containing only normal matter and dark matter.

    And so, scientists invented a new force, called “dark energy”, a sort of anti-gravitational force causing an acceleration in the expansion of the universe out from the big bang 13.8 billion years ago.

    Dark energy isn’t noticeable on small scales, but becomes the dominating force of the universe on the largest cosmic scales: almost four times greater than the gravity of normal and dark matter combined.

    The idea of dark energy isn’t new. Albert Einstein first came up with it to explain a problem he was having when he applied his famous 1915 equations of general relativity theory to the whole universe.

    Like other scientists at the time, Einstein believed the universe was in a steady unchanging state. Yet, when applied to cosmology, his equations showed the universe wanted to expand or contract as matter interacts with the fabric of spacetime: matter tells spacetime how to curve, and spacetime tells matter how to move.

    To resolve the problem, Einstein introduced a dark energy force in 1917 which he called the “cosmological constant”.

    It was a mathematical invention, a fudge factor designed to solve the discrepancies between general relativity theory and the best observational evidence of the day, thus bringing the universe back into a steady state.

    Years later, when astronomer Edwin Hubble discovered that galaxies appeared to be moving away from each other, and the rate at which they were moving was proportional to their distance, Einstein realised his mistake, describing the cosmological constant as the biggest blunder of his life.

    However, the idea has never really gone away, and keeps reappearing to explain strange observations.

    In the mid 1990s two teams of scientists, one led by Brian Schmidt and Adam Riess, and the other by Saul Perlmutter, independently measured distances to Type 1a supernovae in the distant universe, finding that they appeared to be further way than they should be if the universe’s rate of expansion was constant.

    The observations led to the hypothesis that some kind of dark energy anti-gravitational force has caused the expansion of the universe to accelerate over the past six billion years.

    Wiltshire and his colleagues now challenge that reasoning.

    “But these observations are based on an old model of expansion that has not changed since the 1920s”, he says.

    In 1922, Russian physicist Alexander Friedmann used Einstein’s field equations to develop a physical cosmology governing the expansion of space in homogeneous and isotropic models of the universe.

    “Friedmann’s equation assumes an expansion identical to that of a featureless soup, with no complicating structure”, says Wiltshire.

    This has become the basis of the standard Lambda Cold Dark Matter cosmology used to describe the universe.

    “In reality, today’s universe is not homogeneous”, says Wiltshire.

    The earliest snapshot of the universe – called cosmic microwave background radiation – displays only slight temperature variations caused by differences in densities present 370,000 years after the Big Bang.

    However, gravitational instabilities led those tiny density variations to evolve into the stars, galaxies, and clusters of galaxies, which made up the large scale structure of the universe today.

    “The universe has become a vast cosmic web dominated in volume by empty voids, surrounded by sheets of galaxies and threaded by wispy filaments”, says Wiltshire.

    Rather than comparing the supernova observations to the standard Lambda Cold Dark Matter cosmological model, Wiltshire and colleagues used a different model, called ‘timescape cosmology’.

    Timescape cosmology has no dark energy. Instead, it includes variations in the effects of gravity caused by the lumpiness in the structure in the universe.

    Clocks carried by observers in galaxies differ from the clock that best describes average expansion once variations within the universe (known as “inhomogeneity” in the trade) becomes significant.

    Whether or not one infers accelerating expansion then depends crucially on the clock used.

    “Timescape cosmology gives a slightly better fit to the largest supernova data catalogue than Lambda Cold Dark Matter cosmology,” says Wiltshire.

    He admits the statistical evidence is not yet strong enough to definitively rule in favour of one model over the other, and adds that future missions such as the European Space Agency’s Euclid spacecraft will have the power to distinguish between differing cosmology models.

    ESA/Euclid spacecraft

    Another problem involves science’s understanding of Type 1a supernovae. They are not actually perfect standard candles, despite being treated as such in calculations.

    Since timescape cosmology uses a different equation for average expansion, it gives scientists a new way to test for changes in the properties of supernovae over distance.

    Regardless of which model ultimately fits better, better understanding of this will increase the confidence with which scientists can use them as precise distance indicators.

    Answering questions like these will help scientists determine whether dark energy is real or not – an important step in determining the ultimate fate of the universe.

    See the full article here .

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  • richardmitnick 7:10 am on March 1, 2016 Permalink | Reply
    Tags: , , , Cosmological constant,   

    From AAAS: “Conditions for life may hinge on how fast the universe is expanding” 

    AAAS

    AAAS

    Feb. 29, 2016
    Ilima Loomis

    Neutron star merger depicted Goddard
    Neutron star mergers like this produce gamma ray bursts—beams of radiation that can destroy planets or make them inhospitable to life.
    NASA/GSFC

    Scientists have known for several years now that stars, galaxies, and almost everything in the universe is moving away from us (and from everything else) at a faster and faster pace. Now, it turns out that the unknown forces behind the rate of this accelerating expansion—a mathematical value called the cosmological constant—may play a previously unexplored role in creating the right conditions for life.

    That’s the conclusion of a group of physicists who studied the effects of massive cosmic explosions, called gamma ray bursts, on planets.

    They found that when it comes to growing life, it’s better to be far away from your neighbors—and the cosmological constant helps thin out the neighborhood.

    “In dense environments, you have many explosions, and you’re too close to them,” says cosmologist and theoretical physicist Raul Jimenez of the University of Barcelona in Spain and an author on the new study. “It’s best to be in the outskirts, or in regions that have not been highly populated by small galaxies—and that’s exactly where the Milky Way is.”

    Jimenez and his team had previously shown that gamma ray bursts could cause mass extinctions or make planets inhospitable to life by zapping them with radiation and destroying their ozone layer. The bursts channel the radiation into tight beams so powerful that one of them sweeping through a star system could wipe out planets in another galaxy. For their latest work, published this month in Physical Review Letters, they wanted to apply those findings on a broader scale and determine what type of universe would be most likely to support life.

    The research is the latest investigation to touch on the so-called anthropic principle: the idea that in some sense the universe is tuned for the emergence of intelligent life. If the forces of nature were much stronger or weaker than physicists observe, proponents note, crucial building blocks of life—such fundamental particles, atoms, or the long-chain molecules needed for the chemistry of life—might not have formed, resulting in a sterile or even completely chaotic universe. Some researchers have tried to gauge how much “wiggle room” various physical constants might have for change before making the cosmos unrecognizable and uninhabitable. Others, however, question what such research really means and whether it is worthwhile.

    Jimenez and colleagues tackled one, large-scale facet of the anthropic principle. They used a computer model to run simulations of the universe expanding and accelerating at many different speeds. They then measured how changing the cosmological constant affected the universe’s density, paying particular attention to what that meant about gamma ray bursts raining down radiation on stars and planets.

    As it turns out, our universe seems to get it just about right. The existing cosmological constant means the rate of expansion is large enough that it minimizes planets’ exposure to gamma ray bursts, but small enough to form lots of hydrogen-burning stars around which life can exist. (A faster expansion rate would make it hard for gas clouds to collapse into stars.)

    Jimenez says the expansion of the universe played a bigger role in creating habitable worlds than he expected. “It was surprising to me that you do need the cosmological constant to clear out the region and make it more suburbanlike,” he says.

    Beyond what they reveal about the potential for life in our galaxy and beyond, the findings offer a new nugget of insight into one of the biggest puzzles in cosmology: why the cosmological constant is what it is, says cosmologist Alan Heavens, director of the Imperial Centre for Inference and Cosmology at Imperial College London.

    In theory, Heavens explains, either the constant should be hundreds of orders of magnitude higher than it appears to be, or it should be zero, in which case the universe wouldn’t accelerate. But this would disagree with what astronomers have observed. “The small—but nonzero—size of the cosmological constant is a real puzzle in cosmology,” he says, adding that the research shows the number is consistent with the conditions required for the existence of intelligent life that is capable of observing it.

    Lee Smolin, a theoretical physicist at Perimeter Institute for Theoretical Physics in Waterloo, Canada, and a skeptic of the anthropic principle, says the paper’s argument is a novel one and that on first reading he didn’t see any obvious mistakes. “I’ve not heard it before, so they’re to be praised for making a new argument,” he says.

    However, he adds, all truly anthropic arguments to date fall back on fallacies or circular reasoning. For example, many tend to cherry-pick by looking only at one variable in the development of life at a time; looking at several variables at once could lead to a different conclusion.

    Jimenez says the next step is to investigate whether gamma ray bursts are really as devastating to life as scientists believe. His team’s work has shown only that exposure to such massive bursts of radiation would almost certainly peel away a planet’s protective ozone layer. “Is this going to be catastrophic to life?” he says. “I think so, but it may be that life is more resilient than we think.”

    See the full article here .

    The American Association for the Advancement of Science is an international non-profit organization dedicated to advancing science for the benefit of all people.

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