From University of Melbourne (AU): “Exploring the most unknown universe”


From University of Melbourne (AU)

22 December 2020

We have powerful telescopes that can see very old galaxies and the beginnings of new ones. Credit: NASA.

We know how the universe began, yet the data we have amassed in both particle physics and astronomy tells us that we still only know about a tiny fraction of the universe.

Looking at the sky, we observe that on the largest scales, matter is organised into galaxies and clusters of galaxies.

Galaxies contain stars, planets and gases. All the visible universe – the Earth, Sun, stars and galaxies, everything that makes ‘us’ – is made of protons, neutrons and electrons bundled together into atoms.

Physicists have developed an impressive body of knowledge about the fundamental particles and forces that characterise the ordinary matter around us.

Centuries of discovery have shown that apples fall from trees for the same reason that the Earth orbits the Sun, and that the Earth is not at the centre of the universe.

It is a simple and elegant picture.

The quest to answer the most basic questions about the universe has reached a singular moment. Astrophysical and cosmological observations have revealed that our picture of the universe is incomplete.

Dark matter’s fingerprints appear only when we look to the sky at the galactic and super-galactic scales. Picture: Getty Images.

Perhaps one of the most surprising discoveries of the twentieth century was that ordinary matter makes up less than five per cent of the mass of the universe.

The rest of the universe appears to be made of a mysterious, invisible substance named Dark Matter (25 per cent), and a force that repels gravity known as Dark Energy (70 per cent).

Dark Matter Background
Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http://

Coma cluster via NASA/ESA Hubble.

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science).

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL).

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970.

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova.

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.

The exotic unknown substance named dark matter doesn’t appear to absorb, reflect or emit light, rendering it ‘invisible’.

Since dark matter interacts very weakly with normal matter, its existence is inferred due to its gravitational effects on galaxies. Its fingerprints appear only when we look to the sky at the galactic and super-galactic scales, at about 10 million times the distance between the Earth and the Sun.

According to physics, stars at the edges of a spinning, spiral galaxy should travel much more slowly than those near the galactic centre, where a galaxy’s visible matter is concentrated. But our observations show that stars orbit at almost the same speed regardless of their distance from the centre of the galaxy.

This puzzling result only makes sense if we assume that the boundary stars are feeling the gravitational effects of an unseen mass, a halo of dark matter enveloping the galaxy.

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

The existence of dark matter could also explain some optical illusions that astronomers see in the deep universe. For example, there are pictures displaying strange rings and arcs of light.

Installation view of Alicja Kwade’s work WeltenLinie 2020 on display in NGV Triennial 2020 from 19 December 2020 – 18 April 2021 at NGV International, Melbourne © Alicja Kwade, courtesy König Galerie, Berlin. Credit: Tom Ross.

These can be explained if the light from distant galaxies is distorted and magnified by massive, invisible clouds of dark matter, in a phenomenon known as gravitational lensing.

The radical conclusion that the universe is filled with invisible and nearly undetectable matter can be compared to Copernicus’s identification that the Earth is not the centre of the solar system – we have established that we don’t know what most of the universe is made of.

Just like ordinary matter, dark matter also originated from the big bang.

The amount of dark matter produced in the big bang has influenced the evolution of our universe.

Similar to ordinary matter, dark matter isn’t distributed across the universe evenly. Areas of the universe that had slightly more dark matter, and so more gravitational pull, attracted more matter, leading to the creation of the first stars and galaxies.

The cosmic web we observe today is a snapshot of the dark matter distribution in the early universe – this mysterious dark matter acted as a hidden hand guiding the growth of galaxies.

Exploration of this unknown ‘new’ universe necessitates the discovery of the laws of physics underpinning the fundamental particle nature of dark matter. While astrophysical observations study the macroscopic properties of the universe to infer the existence of dark matter, terrestrial physics experiments are essential to study its quantum properties and consequently the fundamental laws of nature associated to dark matter.

The only way to detect dark matter collisions is to place our discovery machines deep underground. Picture: Supplied.

One leading hypothesis is that dark matter is made up of exotic particles that don’t interact much with ordinary matter but are massive and therefore exert a gravitational pull.

Under this assumption, we can derive some of the characteristics of dark matter.

Cosmological measurements indicate that it is ‘cold’ – that is, the dark matter particles are heavy and move around the galaxy very slowly, much slower than the speed of light.

Dark matter very rarely interacts with normal matter and is invisible to light and other forms of electromagnetic radiation, making it impossible to detect with current instruments. To build scientific instruments in order to ‘see’ dark matter, we need to hypothesise about what dark matter is.

Most dark matter has probably been in existence since the first instant of the universe, outdating all atoms that make up visible matter. Earth is perpetually flying through a diffuse cloud of this mysterious substance.

We face a constant shower of dark matter particles – every second, hundreds of thousands of dark matter particles zip through our bodies. However, dark matter particles interact so weakly that they pass right through us, leaving almost no sign of their visit.

Looking at the density of dark matter throughout the universe, scientists calculate that of the thousands of dark matter particles passing through a human body every second, only about a dozen of them will collide with atoms in the body each year.

Installation view of Alicja Kwade’s work WeltenLinie 2020 on display in NGV Triennial 2020 from 19 December 2020 – 18 April 2021 at NGV International, Melbourne © Alicja Kwade, courtesy König Galerie, Berlin. Credit: Tom Ross.

We cannot say what dark matter is until we can study it in a terrestrial laboratory. However, our very limited knowledge about the particle nature of dark matter makes building machines to discover it very challenging.

That is an adventure on its own.

Sometimes, very rarely indeed, a dark matter particle will collide with the nucleus of an atom, which can be detected in carefully designed direct detection experiments.

The expected number of collisions of massive dark matter particles with nuclei of suitable detector materials is very small – less than one collision a year per kilogram of material.

These collisions are so rare that on the Earth’s surface they are drowned out by the billions of cosmic ray–induced collisions that occur in the same kilogram of material each day.

Looking for the signal produced by dark matter collisions is therefore like looking for a needle in a haystack.

The only way to reduce the cosmic ray–induced collisions to a level where dark matter collisions are detectable is to place our discovery machines in deep underground sites where we can use the overlying rock layers (or ‘overburden’) as a shield against cosmic ray radiation.

DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB (CA) deep in Sudbury’s Creighton Mine

LBNL LZ Dark Matter project at SURF, Lead, SD, USA.

Edelweiss Dark Matter Experiment, located at the Modane Underground Laboratory in France

DAMA LIBRA Dark Matter Experiment, 1.5 km beneath Italy’s Gran Sasso mountain located in the Abruzzo region of central Italy.

U Washington Lux Dark Matter 2 at SURF, Lead, SD, USA

About one kilometre underground, far from the cosmic ray–induced particles, our discovery machines are waiting to catch dalliances of these elusive cosmic messengers with ordinary matter. From the depths of a mine shielded from cosmic rays, we can get a glimpse of one of the deepest mysteries of the universe.

Experimental particle physics allows humankind to explore the secret of the universe at its fundamental level and to build machines that will enable this exploration.

We have come to understand the fundamental building blocks of ordinary matter, and what we know of the universe is only a tiny fraction of what is out there.

We know only five per cent of the universe. The remaining 95 per cent is still a mystery – an unknown universe of new particles and forces awaits discovery.

Even if these unknown particles and forces are, at present, invisible to us, they have shaped the universe as we see it today. We are taking part in not only a scientific revolution, but also a revolution in how human beings see the universe.

See the full article here .

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The University of Melbourne (AU) (informally Melbourne University) is an Australian public research university located in Melbourne, Victoria. Founded in 1853, it is Australia’s second oldest university and the oldest in Victoria. Times Higher Education ranks Melbourne as 33rd in the world, while the Academic Ranking of World Universities places Melbourne 44th in the world (both first in Australia).

Melbourne’s main campus is located in Parkville, an inner suburb north of the Melbourne central business district, with several other campuses located across Victoria. Melbourne is a sandstone university and a member of the Group of Eight, Universitas 21 and the Association of Pacific Rim Universities. Since 1872 various residential colleges have become affiliated with the university. There are 12 colleges located on the main campus and in nearby suburbs offering academic, sporting and cultural programs alongside accommodation for Melbourne students and faculty.

Melbourne comprises 11 separate academic units and is associated with numerous institutes and research centres, including the Walter and Eliza Hall Institute of Medical Research, Florey Institute of Neuroscience and Mental Health, the Melbourne Institute of Applied Economic and Social Research and the Grattan Institute. Amongst Melbourne’s 15 graduate schools the Melbourne Business School, the Melbourne Law School and the Melbourne Medical School are particularly well regarded.

Four Australian prime ministers and five governors-general have graduated from Melbourne. Nine Nobel laureates have been students or faculty, the most of any Australian university.