Dark matter heating: a hot topic for the University of Surrey

Dark matter heating: a hot topic for the University of Surrey
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Professor Justin Read, head of the Department of Physics at the University of Surrey, speaks to SciTech Europa about the results of a new study on how dark matter can be ‘heated up’.

In new research, scientists from the University of Surrey, UK, Carnegie Mellon University, USA, and ETH Zürich, Switzerland, have found evidence that dark matter can be heated up and moved around, as a result of star formation in galaxies. The findings provide the first observational evidence for the effect known as ‘dark matter heating’, and give new clues as to what makes up dark matter.

SciTech Europa spoke to Professor Justin Read, lead author of the study and head of the Department of Physics at the University of Surrey, about what the research involved and how the results could help to gain a better understanding of what dark matter – which is thought to make up most of the mass of the universe but, because it doesn’t interact with light in the same way as normal matter, can only be observed through its gravitational effect – actually is.

Why were you looking for signs of dark matter in dwarf galaxies?

Dwarf galaxies are the smallest galaxies in the Universe. They are also almost entirely made up of dark matter. This makes them beautiful natural ‘dark matter laboratories’. By mapping out the distribution of dark matter inside dwarfs and comparing this with numerical models, we can learn about the nature of dark matter.

What is ‘dark matter heating’ and how can it be measured? Why is it important to know more about this phenomenon?

The idea of ‘dark matter heating’ dates back to a paper by Julio Navarro in 1996. Dark matter does not have a temperature or pressure so it cannot be ‘heated up’ in the same way that you can heat up air. However, when air gets hot, the molecules of air all move at a higher relative speed. Our latest models for dark matter suggest that it is made up of some sort of weakly interacting massive particle (WIMP). This dark matter forms an extended shroud around galaxies that we call the dark matter ‘halo’. The dark matter particles in this halo move in orbits, constantly whizzing past one another. If we can raise the relative velocity of these dark matter particles, causing the halo to expand, then this is analogous to heating molecules of air in a room. This is what we mean by ‘dark matter heating’.

Since dark matter interacts with normal matter only via gravity, ‘heating it up’ is quite challenging. However, nature seems to have found a way to do this. At the centres of dwarf galaxies, gas is constantly in a cycle of inflow and outflow. The inflow is caused by cooling. The gas cools, reaches high density and then forms stars. The outflow occurs because the most massive of these stars then produce powerful winds and supernovae explosions that expel gas from the galaxy in a galaxy-wide ‘wind’. The repeated inflow and outflow of gas causes the gravitational potential at the centre of the dwarf galaxy to fluctuate. The dark matter particles feel this constantly changing gravitational force, and their kinetic energy is raised as a result, causing the dark matter particles to shift to higher energy orbits and leading to a global expansion of the dark matter halo.

To help understand this ‘dark matter heating’, imagine that I change the mass of the Sun up and down every two years or so. As the Earth moves along its orbit, and the Sun increases in mass, the Earth would suddenly feel more gravitational force, pulling it closer to the Sun. But then as the Sun reduces in mass, the Earth would find that it is moving too fast. The pull from the Sun is now too weak and the Earth would be flung outwards. The net result is that the Earth would move onto an elliptical orbit like that of Pluto, spending more of its time further away from the Sun. The Earth’s orbit will have been ‘heated up’ and pushed out. This is exactly what happens to the orbits of dark matter particles in dwarf galaxies as a result of repeated gas inflow/outflow over many billions of years.

What relationship did you find between the amount of dark matter at the centres of the dwarfs galaxies and the amount of star formation they have experienced over their lives? Why is this important?

We wanted to use observations of dwarf galaxies to test the above idea that dark matter can be ‘heated up’. A key prediction of the ‘dark matter heating’ model is that dwarf galaxies that experienced very little star formation (i.e. in which star formation ceased over 10 billion years ago) should have undergone little dark matter heating because they experienced only a short period of gas inflow and outflow. These are called ‘quenched’ dwarfs. By contrast, dwarf galaxies that are still forming stars today will have had the full age of the Universe (13.8 billion years) of repeated gas inflow and outflows due to star formation. These star forming dwarfs will have maximised their ability to ‘heat up’ their dark matter, and they should have lower central dark matter density than the quenched dwarfs.

We set out to test the above prediction by measuring the central dark matter density in a sample of 16 nearby dwarf galaxies with excellent quality data and – crucially – a wide range of different star formation histories. Some of our dwarfs stopped forming stars over 10 billion years ago (quenched dwarfs); others are still forming stars today (star forming dwarfs). We found that the quenched dwarfs have a systematically higher dark matter density than the star forming dwarfs, exactly as the dark matter heating models predicted.

This result is important because if dark matter can be ‘heated up’ in the above way, then it suggests that it does indeed comprise some sort of weakly interacting massive particle that can have its orbit altered by a fluctuating gravitational potential. This takes us one step closer to understanding what dark matter is.

Do you have any plans to expand on these discoveries? Will you be relying on (European) infrastructure (ESO instruments, JWST etc.) for future research?

We are following up on this research in several directions. We have a paper submitted and under review measuring the inner dark matter density for a quenched dwarf that lies much further from the Milky Way than any of the quenched dwarfs studied in our current work. This is important to test whether the distance from the Milky Way is somehow a confounding factor in our analysis (we find that it isn’t). We would also like to apply our analysis to data for more dwarfs in the Milky Way and in orbit around our nearby large spiral companion galaxy, Andromeda.

Finally, we can try to measure the inner dark matter density in dwarf galaxies at high redshift to see if these are denser than similar dwarfs in the nearby Universe – another key prediction of ‘dark matter heating’ models. All of this work will be facilitated by new data from the ELT, LSST, Gaia, JWST and other up-coming large surveys.

Professor Justin Read

Head, Department of Physics

University of Surrey

+44 (0)1483 683479


Tweet @PhysicsatSurrey

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  1. The dark matter is ‘pushed out’ because it is a supersolid that fills ’empty’ space and is displaced by ordinary matter.

    ‘Pushed out’ dark matter is curved spacetime.

    Dark matter is a supersolid that fills ’empty’ space, strongly interacts with ordinary matter and is displaced by ordinary matter. What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the supersolid dark matter. The state of displacement of the supersolid dark matter is gravity.

    The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.

    Displaced supersolid dark matter is curved spacetime.

  2. “What we see is only a tiny portion of what is.”
    That is the point! To say “What is” belongs to metaphysics! So, let’s look at it as metaphysics would.

    What “is” is something that exists by itself and this is called a “substance”. A “substance” is what we interact with to produce an “experience”. In other words, the universe is made of substance and our reality is the sum of all of our experiences of the substance. Since the substance exists by itself, it does not require our experience of it in order to exist! This means that the substance may be found in a simple state or form of which we have no “experience”. We still may deduce the existence of a substance from experience. To “deduce” is not a direct experience.

    The universe is a logical system. This is because the universe obeys the rule of non-contradiction and logic based mathematics are very effective in describing how it works. Such a logical system requires, in order to be operational, that it be all made of only one type of substance.

    “Time” is the one thing we can only deduce from the experience of change. So, the universe is all made of this dynamic process we will call the time-process. All experiences looking for dark matter only deduce its presence “indirectly” from the clues of the effects it produces. What I am saying is that those effects can exist without any matter causing them because the time-process is a substance which can influence itself and cause these effects on its own.

    Matter replaces the time-process, slows it down (the effect), which is what we call gravity. The only way this local replacement of time can have a non-local effect is if this time-process transmits (influence) this time-process deficit away from close to close. This time-process can influence itself, clump together etc.

    In other words, we use some effects on time to infer the presence of some matter or mass when the “effect” itself is just a characteristic of the time-process as a substance. This time-process makes everything in the universe as planets, stars, galaxies and all that is in between.

    Unless they call it “time-process”, they will NEVER find any dark matter. There is just no place for anything else in a logical universe than the time-process substance and its different forms like matter


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