Cosmic giants shed new light on dark matter

Jun 13, 2013
Dark matter maps for 50 individual galaxy clusters (left), the average galaxy cluster (centre), and based on dark matter theory (right). The CDM theory (right, centre) is a close match with the average galaxy cluster observed with the Subaru telescope. The density of dark matter increases in the order of blue, green, yellow, red, and black colors. Credit: University of Birmingham

(Phys.org) —Astronomers at the University of Birmingham (UK), Academica Sinica in Taiwan, and the Kavli Institute of Physics and Mathematics of the Universe in Japan, have found new evidence that the mysterious dark matter that pervades our universe behaves as predicted by the 'cold dark matter' theory known as 'CDM'.

At a press conference today in Taipei the team of astronomers report their measurements of the density of dark matter in the most massive objects in the universe, namely . They found that the density of dark matter decreases gently from the centre of these cosmic giants out to their diffuse outskirts. The fall in dark matter density from the centre to the outskirts agrees very closely with the CDM theory.

Almost eighty years after the first evidence for dark matter emerged from , few scientists seriously doubt that it exists. However astronomers cannot see dark matter directly in the night sky, and have not yet identified the dark matter particle in their experiments.

"What is dark matter?" is therefore a big unanswered question facing astronomers and particle physicists, especially because there is strong evidence that 85% of the mass in the universe is invisible dark matter.

The team, led by Dr Nobuhiro Okabe (Academia Sinica) and Dr Graham Smith (Birmingham), used the in Hawaii to investigate the nature of dark matter by measuring its density in fifty galaxy clusters, the most massive objects in the Universe.

"A galaxy cluster is like a huge city that you view from above during the night', explains Smith. 'Each bright city light is a galaxy, and the dark areas between the lights that appears to be empty during the night are actually full of dark matter. You can think of the dark matter in a galaxy cluster as being the infrastructure within which the galaxies live. We wanted to know how the density of dark matter changes as you drive from the centre of a these huge cities out to the suburbs."

An outline of how the dark matter distribution (right) was reconstructed from optical images (left) taken by the Subaru Telescope. A precise measurement of the shapes of background galaxies in observed images enabled the team to investigate the distortion pattern (center) and then reconstruct the distribution of dark matter in the galaxy clusters. Credit: NAOJ/ASIAA/School of Physics and Astronomy, University of Birmingham/Kavli IPMU/Astronomical Institute, Tohoku University

The density of dark matter depends on the properties of the individual dark matter particles, just like the density of everyday materials depends on what they are made of. CDM - the most successful dark matter theory to date - predicts that dark matter particles only interact with each other and with other matter via the force of gravity, they don't emit or absorb light. It also predicts that the density of dark matter in the centre of the most massive objects in the universe, for example the Coma cluster of galaxies - the nearest cosmic giant to Earth - is lower than in less massive objects, including individual galaxies like our own home, the Milky Way.

Observing galaxy clusters with the Subaru Prime Focus Camera (Suprime-Cam), the team measured the density of dark matter in galaxy clusters using the effect of gravitational lensing. As predicted by Einstein, gravitational lensing is the change in the direction and shape of a light beam as it travels through the curved space close to a massive object. The apparent shape and position of distant galaxies that astronomers observe are therefore altered by the dark matter in cosmic giants.

Lead author Okabe enthused, 'The Subaru Telescope is fantastic machine for gravitational lensing. It allows us to measure very precisely how the dark matter in galaxy clusters distorts light from distant galaxies and gauge tiny changes in the appearance of a huge number of faint galaxies.'

CDM theory uses two numbers to describe how the density of dark matter in galaxy clusters changes from the dense centre to the low density outskirts. One number is simply the mass of the galaxy cluster, and the second is the so-called concentration parameter - CDM theory predicts that galaxy clusters have a low concentration parameter (less dense central regions), in contrast to individual galaxies that have a high concentration parameter (more dense central regions).

The team combined observations of 50 of the most massive known galaxy clusters into a single measurement of the average concentration parameter of massive galaxy clusters. They found that the concentration parameter, that describes how density changes from the center to the outskirts of clusters, matches the CDM theory.

In the past, astronomers have studied small handfuls of clusters, finding that they generally have large concentration parameters, suggesting possible problems with CDM theory. Okabe and Smith suspected that if they used a large number of clusters to measure the average concentration parameter, then they might get a different result. 'We didn't know what we would find', comments Okabe, 'we were curious what we would find, if we took a different approach.'

After several years of observations, and careful data analysis, the results speak for themselves. The concentration parameter of galaxy clusters in the nearby universe matches CDM theory.

Hints that measurements of dark matter in large numbers of galaxy clusters might support CDM theory emerged in 2010 when this team published their preliminary results in the Publications of the Astronomical Society of Japan. They recently won that Society's 2012 Excellent Paper Award for that work, and are now enjoying the fruits of their hard work to improve their analysis and expand their study in the intervening years. "This is a very satisfying result. Our new results are based on a very careful analysis of the best available data," comments Okabe.

What does the future hold for the team's continued research on dark matter? Smith noted 'We don't stop here. For example, we can improve our work by measuring the dark on even smaller scales - right in the centre of these galaxy clusters. Adding measurements on smaller scales will help us to learn more about in the future.'

Team member Professor Masahiro is also excited about the future: "Combining lensing observations of many galaxy clusters into a single measurement like this is a very powerful technique. Japanese astronomers are preparing to use Subaru Telescope's new Hyper Suprime-Cam (HSC) to conduct one of the biggest surveys of galaxies in human history. Our new results are a beautiful confirmation of our plan to use HSC for gravitational lensing studies."

Explore further: POLARBEAR detects curls in the universe's oldest light

More information: The research paper on which this release is based was published online in the May 17, 2013 edition of the Astrophysical Journal Letters: N. Okabe et al., "LoCuSS: The Mass Density Profile of Massive Galaxy Clusters at z=0.2", Volume 769, Number 2, Article ID. 35 (2013). iopscience.iop.org/2041-8205/769/2/L35/

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HannesAlfven
1 / 5 (9) Jun 13, 2013
Re: "After several years of observations, and careful data analysis, the results speak for themselves."

No offense intended to the researchers, who I trust are extremely talented, but one wonders if dark matter researchers will eventually regret not widening the scope of their investigation to also include questioning accepted hypotheses. A person can be forgiven for suspecting that we are at an impasse here, and that the situation will simply not change for the better.

Re: "few scientists seriously doubt that it exists ..."

Perhaps that's actually the problem. If the discipline basically weeds out approaches which question foundational hypotheses, then wouldn't this be an extremely risky strategy in the event that a wrong turn was taken? Many lines of investigation have been tossed out along the way. As each decade passes, the probability that somebody will possess the courage to suggest that all of this investment was a mistake will predictably approach zero.
Q-Star
2.3 / 5 (6) Jun 13, 2013
As each decade passes, the probability that somebody will possess the courage to suggest that all of this investment was a mistake will predictably approach zero.


Where have ya been living? Ya need to get out more, people suggest this all the time. Somebody? Well I can attest that there are more than a few somebodies who possess the courage,, hundreds, nay thousands of them. The one's who have some science in their proposals get heard. The cranks, crackpots, and pseudo guys? They get heard more than is necessary.

Maybe ya should do as I suggested to Zephyr, try to get a "Mainstream Fairness to Bunk Science Act" passed. Compel them to listen, compel them to believe under penalty of law.
cantdrive85
1 / 5 (6) Jun 13, 2013
Where have ya been living? Ya need to get out more, people suggest this all the time.

Halton Arp, that worked well for him.
Ober
5 / 5 (6) Jun 13, 2013
I'd like to point out here, that this article is very good in my opinion. People often think that Dark Matter is a mere fudge factor to make OUR theories of Galactic rotation work according to OUR theories of Gravity. However in this case Dark Matter is implied due to Gravitational Lensing. So in either case Dark Matter is implied through experimental observation, and to the horror of scientists it isn't understood via current models. Now THAT is SCIENCE!!!!
Look back at Particle Physics and you will see discoveries such as the Neutron and Neutrino were once fudge factors as well!!!
So while it may be annoying that we don't know what this Dark Matter/Energy/Flow stuff is, you should be excited that there is still an enormous amount of physics yet to be deciphered. The outcomes of this research will be mind blowing once the mystery is unveiled. Though I suspect we'll just be unwrapping another layer of the onion.
LarryD
5 / 5 (1) Jun 13, 2013
Ober, well said! Just that I wish you had used another veg ref. 'An apple a day keeps the doctor away but an onion a day keeps everybody away'
Urgelt
5 / 5 (1) Jun 14, 2013
Ober wrote, "...Dark Matter is implied due to Gravitational Lensing."

Actually, no. What's implied is the presence of an unseen, distributed source of gravity.

CDM theory produces a match when 50 galaxy clusters are averaged. Individual galaxy clusters generally do not. Studies of individual galaxies thus far generally have not matched CDM theory. This gravitational lensing result is certainly interesting, but CDM theory has a long way to go before we'll conclude that it's correct.

Thus far the only sources of gravity that we can pin down are associated with matter. It's only natural that we should assume that a source of gravity we can *not* pin down is also associated with matter; hence the CDM and other dark matter theories.

But until we pin down the nature of that matter, it's an *assumption* that any form of matter is responsible for these observed gravitational phenomena.

Perhaps it's a likely assumption. But it's still an assumption.
roldor
1 / 5 (3) Jun 14, 2013
When DM causes gravity, then one can assume, that DM is also attracted by gravity. Or not?
If yes, the DM should also form bodies and fall on bodies made from ordinary matter.
Maybe, DM is some kind of "half matter". When it comes together, then it forms normal matter
or a neutron, that decays into H. So the DM cold be a quark-gas, as we define it.
This combining could also contribute to the background-radiation.
roldor
1 / 5 (6) Jun 14, 2013
Hello, also quarks have not been detected yet. When we call matter real, then we could call it maybe as semi-real, or not yet really existing particles. These must have an energie and
therefore a mass with gravitation.
roldor
1 / 5 (1) Jun 14, 2013
From WiPediia:Two terms are used in referring to a quark's mass: current quark mass refers to the mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark.[66] These masses typically have very different values. Most of a hadron's mass comes from the gluons that bind the constituent quarks together, rather than from the quarks themselves. While gluons are inherently massless, they possess energy – more specifically, quantum chromodynamics binding energy (QCBE) – and it is this that contributes so greatly to the overall mass of the hadron (see mass in special relativity). For example, a proton has a mass of approximately 938 MeV/c2, of which the rest mass of its three valence quarks only contributes about 11 MeV/c2; much of the remainder can be attributed to the gluons' QCBE.

They say that a quark-gluon plasma can only exist in extremely high T. But what mechanisms
could lead to a cooling?
Fleetfoot
5 / 5 (5) Jun 14, 2013
Actually, no. What's implied is the presence of an unseen, distributed source of gravity.


Gravity is produced by stress and energy. Matter is only one form of energy.

This gravitational lensing result is certainly interesting, but CDM theory has a long way to go before we'll conclude that it's correct.


Hot versus cold refers to the mean speed of the particles. If they were hot then their speed would mean that they would have smoothed out the early variations in density and the structure of the universe would be very different from what we see.

But until we pin down the nature of that matter, it's an *assumption* that any form of matter is responsible for these observed gravitational phenomena. ... it's still an assumption.


If it is a "gravitational phenomena" within GR then it is produced by some form of energy, and that energy forms clumps so cannot be moving at the speed of light. That means it must have mass. It is matter but of an unknown type.
Fleetfoot
5 / 5 (4) Jun 14, 2013
... and to the horror of scientists it isn't understood via current models. Now THAT is SCIENCE!!!! ...


I think you meant " to the delight of scientists". There's nothing they like more than a mystery and the opportunity to discover something new.
Fleetfoot
5 / 5 (3) Jun 14, 2013
When DM causes gravity, then one can assume, that DM is also attracted by gravity. Or not?


Yes, that is what caused the large scale structure to form in the universe. See this amazing video which shows dark matter flow marked by galaxies:

http://phys.org/n...rse.html

If yes, the DM should also form bodies and fall on bodies made from ordinary matter.


No, since it doesn't interact with EM, it falls straight through normal matter, just like neutrinos.
antialias_physorg
4.1 / 5 (9) Jun 14, 2013
There's nothing they like more than a mystery and the opportunity to discover something new

Exactly. That's why people become scientists in the first place.
(Apart from the tons of money, the short working hours, the immense prestige, the droves of women and the adulation of millions of fans, of course)
Fleetfoot
5 / 5 (2) Jun 14, 2013
They say that a quark-gluon plasma can only exist in extremely high T. But what mechanisms could lead to a cooling.


The usual thermal radiation will cool it easily, but below a threshold temperature, the quarks bind together into hadrons and you no longer have the "soup".
vidyunmaya
1 / 5 (8) Jun 15, 2013
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jaxFrost
not rated yet Jun 15, 2013
"Team member Professor Masahiro is also excited about the future..."

I think the author of this article failed to note that "Masahiro" is actually a given name in Japanese, not a family name!
Neinsense99
3.3 / 5 (7) Jul 06, 2013
Hello, also quarks have not been detected yet. When we call matter real, then we could call it maybe as semi-real, or not yet really existing particles. These must have an energie and
therefore a mass with gravitation.

Don't worry, that boot headed for your posterior hasn't been detected by you yet and is thus only semi-real.