What’s the Universe made of? Don’t worry if you don’t have a clue, astronomers don’t either. The Universe is dominated by a mysterious dark matter that seems to form the true mass of a galaxy, not the regular matter – like stars and planets – that we can actually see. Dr. James Jee from Johns Hopkins University used the Hubble Space Telescope to create a detailed map of dark matter concentrations around two galaxies. And astronomers just got some new clues.
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Fraser Cain: We’ve heard the term dark matter quite a bit. Can you give us the current understanding of what dark matter is?
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Dr. James Jee: Before I speak about dark matter, I have to mention what astronomers now believe how the Universe came into what it is today. We believe that 30% of the Universe is matter, and the other 70% is dark energy, and dark matter comprises more than 90% of matter in the Universe. Nobody has detected dark matter in the labs, so they don’t know the shape, colour, or smell of it, but there is evidence that it’s there. We can detect it by the so called gravitational lensing.
Fraser: So you recently performed a survey using the Hubble Space Telescope to map the concentration of dark matter. What was the process to do that?
Dr. Jee: Dark matter comprises, as I said, 90% of the matter in the Universe, and the best place to look for dark matter is where it abounds the most. So we pointed the Hubble Space Telescope to two of the most interesting galaxy clusters forming when the Universe was half its current age.
Fraser: And what did you see?
Dr. Jee: We examined the spectral distribution of the background galaxies. By examining the distortion of those background galaxies, we were able to determine the density of dark matter in the foreground.
Fraser: Let me see if I understand this correctly. You were looking at distant galaxies, and by seeing the way the light was changing as it came towards us, you were able to detect where there were hidden clumps of matter that was affecting it gravitationally.
Dr. Jee: Exactly. Maybe this is a good analogy. Suppose you’re reading a news article using a magnifying glass, you can infer the power, or the thickness of the lenses by examining how much the letters look larger through the magnifying glass. Similarly, if you look at the distortion, or the magnification, of the background galaxies, you can determine the density of the elusive dark matter in the foreground.
Fraser: So then what is the relationship between dark matter and the galaxies that we can see?
Dr. Jee: It’s the dominant matter in the Universe and it has gravity. Without dark matter, it’s very hard to form galaxies with the large scale structures that we see in today’s Universe. So definitely, dark matter helps the formation of galaxies in the large scale structure.
Fraser: Is it possible, then, that where ever the dark matter is clumping, that’s where we’re likely to see galaxies?
Dr. Jee: Yes, that’s basically what we’ve found in our research. People have speculated that dark matter is collisionless particles and the dark matter and the normal matter should exist together. But no one has been able to determine this very clearly because the dark matter does not emit any electromagnetic waves. What we have found using the Hubble is that the luminous galaxies form at the densest regions of those dark matter halos.
Fraser: If we know that this kind of clumping is happening – the two seem to go hand in hand – does this allow you to throw out any existing theories for what this dark matter might be?
Dr. Jee: Yes, this gives us a lot of clues. Most people believe that dark matter is collisionless, but some suggest that they may have some collision properties like hydrogen gas. The way that dark matter clumps together gives us hints about what the dark matter is. Suppose the dark matter has collision properties, like the hydrogen atom, then they collide with each other very frequently, and we’ll see a very smooth distribution of a dark matter halo. But we have found is that these structures are very clumpy, like the mass of a galaxy itself. That indicates that dark matter particles, if any, will be collisionless particles as most of the theories say in today’s astronomy.
Fraser: Oh, I see, so the actual particles that could be causing this dark matter are either so small, or so weakly interacting that they’re not even bonking together. And if they did bonk together, you would actually see a more even spray of distribution. So then based on the findings you’ve gotten, what would be the next stage of your research?
Dr. Jee: The Advanced Camera for Surveys program encompasses more than 15 galaxy clusters that are very very interesting. These are just the first two results. We believe that if we complete our 15 galaxy clusters for the survey, then we’ll have a more clear picture of how the dark matter and the normal matter interact, possibly by gravity together. And we may have a more clear idea of how the dark matter contributes to the formation of the large scale structure of the Universe.
Fraser: And based on the research you’ve done so far, do you have a pet theory for what the dark matter might be?
Dr. Jee: Well, if you go to the Astro-ph website, that’s the website that people upload their various research papers, and there are like 10 or 15 papers a day. And there are many speculations about this that are very attractive and plausible. But I guess the nature of dark matter may be answered in 10 or 15 years from now, but we are still searching. Our research gives an unprecedented resolution of dark matter can differentiate between collisional and collisionless particles.
Fraser: And are there any other instruments other than Hubble that can do this work?
Dr. Jee: We can do the gravitational lensing using the ground-based telescopes. In fact it was in 1990 that people first detected the dark matter using gravitational lensing. But when you do the gravitational lensing using a ground-based telescope, the resolution is so poor. In other words, the atmospheric turbulence will smear the gravitational lensing so we can’t see a very high quality image of the dark matter. But if we use the telescope in space, then it does not blur the shape of the background image so you conserve the gravitational lensing signal. We can come up with a very high resolution image of dark matter distribution.
Fraser: And a bigger instrument would give you a better picture.
Dr. Jee: Definitely. The next telescope is the JWST (James Webb Space Telescope) will effectively increase the resolution of the significance of dark matter by a factor of 10 or more.
Fraser. Do you think you’d see anything significantly different with the 10x resolution?
Dr. Jee: The global shape of dark matter distribution will not change very much, but in that case we may be able to compare the structure of the dark matter with respect to the galaxies. In that case, we may be able to answer whether the dark matter particles have some collisional properties. In the beginning, I said that what I have found is consistent with the collisionless hypothesis. But there have been some suggestions that dark matter particles may have some very small collisional properties. So we could determine the offset between dark matter and galaxy matter. That gives you a lot of possible constraints on the collisional cross sections between normal and dark matter particles.
This research was reported in Universe Today on December 13, 2005.