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  • richardmitnick 11:18 am on March 13, 2019 Permalink | Reply
    Tags: "Streams of Stars Snaking Through the Galaxy Could Help Shine a Light on Dark Matter", Adrian Price-Whelan calls GD-1 "the Goldilocks stream" because it's in just the right place., , , At about 33000 light-years (10 kiloparsecs) GD-1 is the longest stellar stream in the galactic halo, , , Dark matter makes up the bulk of the mass in the universe but it has never been directly observed, , ΛCDM model, , scores of dark matter seeds are scattered through galaxies like the Milky Way, , The stellar stream known as GD-1 is a thin flow of material tucked inside the Galactic halo   

    From smithsonian.com: “Streams of Stars Snaking Through the Galaxy Could Help Shine a Light on Dark Matter” 

    smithsonian
    From smithsonian.com

    March 12, 2019
    Nola Taylor Redd

    When the Milky Way consumes another galaxy, tendrils of stellar streams survive the merger, containing clues about the universe’s mysterious unseen matter.

    1
    An ultraviolet image of the Andromeda galaxy, the closest major galaxy to the Milky Way, taken by NASA’s Galaxy Evolution Explorer space telescope. Like our own galaxy, Andromeda is a spiral galaxy with a flat rotating disk of stars and gas and a concentrated bulge of stars at the center. (NASA/JPL-Caltech)

    When a small galaxy strays too close to the Milky Way, the gravity from our larger galaxy reels it in. Gas and stars are ripped from the passing galaxy as it falls inward toward its doom, creating streams of material that stretch between the galactic pair. These streams continue to tear away stars until the infalling object has been completely consumed. After the merger is over, some of the only remaining signs of the devoured object are the stellar streams snaking through the Milky Way, a small sample of stars from a galaxy long gone.

    In addition to being a record of the past, one of these streams may provide the first direct evidence for small scale clusters of dark matter—the elusive material that is believed to account for 85 percent of all matter in the universe. A recent analysis of a trail of stars reveals that it interacted with a dense object in the last few hundred million years. After ruling out the most likely suspects, the researchers determined that the relatively recently made gap in the stream may have been caused by a small clump of dark matter. If confirmed, the eddies of this stellar stream could help scientists sort through the competing theories about dark matter and perhaps even close in on the characteristics of the mysterious material.

    The stellar stream known as GD-1 is a thin flow of material tucked inside the Galactic halo, the loose collection of stars and gases surrounding the disk of the Milky Way. Using data released last April from the European Space Agency’s Gaia space telescope, which is in the process of assembling the most detailed map of the Milky Way’s stars ever made, astronomers were able to use precise positional data to reconstruct the movement of the stars in GD-1.

    ESA/GAIA satellite

    Torn from a cloud of material, the stream is the last remnant of an object that was likely consumed by our galaxy in the last 300 million years—an eyeblink on astronomical timescales.

    Gaia found two small breaks in the stream, the first unambiguous observation of gaps in a stellar stream, as well as a dense collection of stars called a spur. Together, these features suggest that a small but massive object shook up the material of the stream.

    “I think this is the first direct dynamical evidence for the small-scale [structure] of dark matter,” says Adrian Price-Whelan, an astronomer at the Flatiron Institute in New York. Working with Ana Bonaca of the Harvard-Smithsonian Center for Astrophysics, Price-Whelan investigated the newfound structures in GD-1 to determine their source and presented the results earlier this year at the winter meeting of the American Astronomical Society.

    At about 33,000 light-years (10 kiloparsecs), GD-1 is the longest stellar stream in the galactic halo. While Price-Whelan and his colleagues were able to use models to show that one of the gaps formed during the generation of the stream, the other gap remained a mystery. However, along with the puzzle, Gaia also revealed a solution: the spur.

    When an object travels past or through a stellar stream, it disrupts the stars. Price-Whelan compares the disruption to a strong jet of air blowing across a stream of water. The water—or stars—plume outward along the path of the disruptor, creating a gap. Some move so fast that they escape the stream and go flying off into space, lost forever. Others are pulled back into the stream to form eddy-like features astronomers call spurs. After a few hundred million years, most spurs merge back into the stream, and only the gap remains, though some can be longer-lived.

    When it comes to spotting structures in stellar streams, Price-Whelan calls GD-1 “the Goldilocks stream” because it’s in just the right place. GD-1 is within the stars of the Milky Way, but moving in the opposite direction, making it easier for astronomers to pick out the stars in the stream from the surrounding objects. “At any given location, it’s moving differently from the way most of the other stars in that part of the sky are moving,” Price-Whelan says.

    The researchers modeled what type of objects could be responsible for the relatively newborn spur spotted in GD-1. They determined that the responsible object had to weigh in with a mass somewhere between 1 million and 100 million times the mass of the sun. Stretching only about 65 light-years (20 pc) in length, the object would have been incredibly dense. The interaction between the stream and the dense object would have likely happened within the last few hundred million years out of the 13.8-billion-year lifetime of the universe.

    Milky Way NASA/JPL-Caltech /ESO R. Hurt

    Dark matter isn’t the only object that could have disrupted the stellar stream. A globular cluster or dwarf galaxy swooping nearby could also have created the gap and spur. Price-Whelan and his colleagues turned their eyes toward all known such objects and calculated their orbits, finding that none came close enough to GD-1 in the last billion years to shake things up. A chance encounter with a primordial black hole could have sent the stream’s stars flying, but it would have been an extremely rare event.

    According to dark matter simulations that allow for small structures, scores of dark matter seeds are scattered through galaxies like the Milky Way. A stream like GD-1 is expected to encounter at least one such seed within the last 8 billion years, making dark matter a far more likely perturber based on encounter rates than any other object.

    Dark matter makes up the bulk of the mass in the universe, but it has never been directly observed. The two leading theories for its existence are the warm dark matter model and the Lambda cold dark matter model (ΛCDM), which is the model preferred by most scientists.

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex Mittelmann Cold creation

    Under ΛCDM, dark matter forms clumps that can be as large as a galaxy or as small as a soda can. Warm dark matter models suggest that the material has less massive particles and lacks the can-sized structures that the ΛCDM model suggests. Finding evidence for small scale structures of dark matter could help weed out certain models and start to narrow in on some of the characteristics of the tantalizing stuff.

    “Streams might be the only avenue that we could [use to] study the lowest mass end of what dark matter is doing,” Price-Whelan says. “If we want to be able to confirm or reject or rule out different theories of dark matter, we really need to know what’s happening at [the low] end.”

    Gaia’s data helped identify the stars of the spur, but it’s not detailed enough to compare the velocity differences between them and the stars in the stream, which could help confirm that dark matter perturbed the structure. Price-Whelan and his colleagues want to use NASA’s Hubble Space Telescope to further study the movement of the faint stars in GD-1. Although Gaia has opened the door to wide-scale examination of the movement of stars across the Milky Way, Price-Whelan says that it can’t compete with the HST when it comes to very faint stars. “You can drill much deeper when you have a dedicated telescope like Hubble,” he says.

    The differences in how the stars of the stream and spur move could help astronomers determine how much energy the perturbing object carried, as well as allow researchers to calculate its orbit. These pieces of information could be used to track down the disruptive dark matter clump and study its immediate environment.

    In addition to making a more in-depth study of GD-1, astronomers plan to apply the same techniques enabled by Gaia’s data to some of the more than 40 other streams surrounding the Milky Way. Spotting spurs and gaps in other streams and tying them to dark matter could further improve our understanding of how the mysterious substance interacts with the visible galaxy.

    After decades of puzzling over the mystery of dark matter, the gaps and spurs in stellar streams like GD-1 may finally help to reveal the secrets of the substance that makes up most of the universe. “This is one of the most exciting things that has come out of Gaia,” Price-Whelan says.

    See the full article here .

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  • richardmitnick 12:19 pm on October 5, 2017 Permalink | Reply
    Tags: , , BAO (SDSS-IV) survey, , , , ΛCDM model, , The mysterious repulsive force known as dark energy   

    From Science Alert: “Mysterious Dark Energy Is More Dynamic Than We Thought, Says New Study” 

    ScienceAlert

    Science Alert

    4 OCT 2017
    MIKE MCRAE

    1
    betibup33/Shutterstock

    Not so constant after all.

    For over 20 years, physicists have puzzled over why space appears to be flying apart at the seams.

    New research is adding some deeper insight into the mysterious repulsive force known as dark energy, providing evidence that whatever it might be, its ghostly influence hasn’t been constant over time.

    In 2016, an international team of researchers accurately measured fluctuations in the density of visible matter through the Universe over long periods of its history.

    These shifts – called baryon acoustic oscillations (BAO) – provide something of a yardstick for cosmologists studying relative distances over time.

    Just as astronomers have used light from distant exploding stars to conclude the Universe is spreading out, cosmologists (big picture astronomers) have used BAO.

    Whichever of these two tools we use, it looks as if the Universe has been gaining real estate over the 13.82 billion years of its existence, causing clumps of material in it to spread out.

    Weirder yet, that growth has been speeding up for quite some time.

    The unit used to describe this expansion is called the Hubble Constant, and is thought to be the result of the tension between matter pulling itself together and the swelling of space in between.

    Why is space growing? Nobody is really all that certain, and that’s a problem.

    To help come up with an explanation astrophysicists look at the hum of empty space as if it has qualities, and isn’t just an empty stage for fields and particles.

    The odds-on favourite description at the moment is called the Lambda Cold Dark Matter (ΛCDM) model, which combines what’s referred to as the Friedmann-Lemaitre-Robertson-Walker (FLRW) model of empty space with a distribution of visible and invisible matter within it.

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    In this model, dark energy is the constant push of emptiness between masses, possibly caused by the hiss of particles popping in and out of virtual existence.

    But the ΛCDM model is built on a number of assumptions, leaving open the question; does dark energy need to be among the fundamental qualities of space, static over time?

    Or could it be influenced by its surroundings, shifting as the Universe changes?

    “Since its discovery at the end of last century, dark energy has been a riddle wrapped in an enigma,” says researcher Bob Nichol from the Institute of Cosmology and Gravitation (ICG) at the University of Portsmouth.

    “We are all desperate to gain some greater insight into its characteristics and origin.”

    Armed with more accurate measures of these tides of matter pulsing like a cosmic heartbeat, the researchers applied their BAO data to a dark energy model developed by Gong-Bo Zhao, from the University of Portsmouth and the National Astronomical Observatories of China (NAOC).

    The study’s results point to a more dynamic description of this mysterious force.

    This conclusion is based in part on a conflict between data produced by the team’s own BAO survey and interpretations based on the cosmic microwave background (CMB) – the echo of light bouncing through the Universe since moments after the Big Bang.

    CMB per ESA/Planck


    ESA/Planck

    This diagram on the BAO (SDSS-IV) survey gives you some idea of how it relates to the CMB.

    4
    Sloan Digital Sky Survey

    One way the researchers found they could resolve this difference is to treat dark energy as if it is dynamic changing with time.

    If true, it would mean dark energy isn’t a force produced by the bubbling of a vacuum.

    The significance of their results isn’t enough to overturn the evidence favouring the static dark energy feature of the ΛCDM model.

    But that could all change with data collected from the Dark Energy Spectroscopic Instrument when it starts its survey next year.

    “We are excited to see that current observations are able to probe the dynamics of dark energy at this level, and we hope that future observations will confirm what we see today,” says Zhao.

    Whatever the outcome, it’ll be worth it – the fate of the Universe is at stake, after all.

    This research was published in Nature Astronomy.

    See the full article here .

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