May 28 2021

New Dark Matter Map Mystery

Published by under Astronomy
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Scientists have published the most extensive map of dark matter in the universe to date, based on a survey of 100 million galaxies. The findings don’t quite match with predictions made by computer models, suggesting that there is some physics at work which scientists do not yet understand. This, of course, is exciting for physicists.

As I discussed previously, we don’t know what dark matter is, but we are pretty confident it’s there. Dark matter does not give off any radiation, but it does have gravity, so we can see its gravitational effects. Based on these observations it seems that 80% of the matter in the universe is dark matter. This is a major area of research, because we do not know what dark matter is made of. It is probably some new particle we have not identified so far. This is where scientists live – on the edge of our current knowledge, peering into the unknown.

Part of that “peering” is gathering lots of data, and that is what the current study does. They used gravitational lensing to map the gravity of the universe, 80% of which is dark matter. Visible galaxies and dark matter cluster together, creating an overall structure to the universe. There are vast black voids with nothing, and there are tendrils of matter with galaxies, gas, and stars. The goal is to map this distribution, to see where all the stuff in the universe is.

They then compared this map to what we would predict based on our current understanding of the laws of physics. They started with a map of where all the matter was 350,000 years after the Big Bang, which was created by examining the cosmic background radiation. Then they model where that matter should have gone over the last 13.8 billion years based upon relativity and other physical laws. The map and the model were off by a few percent. The universe is more evenly distributed than the models predict. This may not sound like a lot, but physicists are used to dealing with high levels of precision. Physical laws tend to be very reliable. This is why we can make calculations and send a probe to Pluto 5 billion km away, and arrive precisely where they predicted. If the New Horizons probe was off course by a few percent, that would have been a disaster, both for the mission and our understanding of the relevant laws of physics.

This is why physicists love discrepancies between predicted and observed phenomena, even tiny ones. It means something is going on we are not aware of. This could be an effect we have not considered, an error in their experimental design or method of observation, or occasionally a tweak to our understanding of the laws of physics. The first two need to be thoroughly ruled out before new physics can be confidently postulated, and it is an increasingly rare event, but that is what physicists live for.

Remember the Pioneer Anomaly – there was a slight difference in the acceleration of the Pioneer probes from what scientists predicted. This teeny tiny discrepancy was the subject of great examination and consideration. Eventually it was solved, and turned out to be a slight asymmetry in the radiation pressure from the heat generated by the probes.

The current dark matter map anomaly, being off by a few percent, is huge by comparison. Again, the first two possibilities need to be considered. Perhaps we are not accounting for something in the universe. Perhaps there was an error in the model, Or perhaps either the CBR measurements or the dark matter map is off for some reason. New independent observations, and running everything through review of the physics community, will deal with these issues. At this point it’s too early to tell. But if everything holds up, then that third possibility – new physics – is the reward.

The BBC quotes one of the authors:

Dr Niall Jeffrey, of École Normale Supérieure, in Paris, who pieced the map together, said that the result posed a “real problem” for physics.

“If this disparity is true then maybe Einstein was wrong,” he told BBC News. “You might think that this is a bad thing, that maybe physics is broken. But to a physicist, it is extremely exciting. It means that we can find out something new about the way the Universe really is.”

Dr. Frenk, who worked on the model that now appears to be challenged, was also quoted:

“I spent my life working on this theory and my heart tells me I don’t want to see it collapse. But my brain tells me that the measurements were correct, and we have to look at the possibility of new physics. Then my stomach cringes, because we have no solid grounds to explore because we have no theory of physics to guide us. It makes me very nervous and fearful, because we are entering a completely unknown domain and who knows what we are going to find.”

These quotes I think capture the typical reaction of scientists to such things. Sure, it’s destabilizing, perhaps disappointing, and even a little scary. But they can’t deny the data if it is reliable, and at the end of the day, excitement about the possibility of new discovery takes over. Also, younger scientists are less invested in the older models, and more anxious to break them with new discoveries. What you typically don’t see is denial and cover-up in order to protect the “status quo”. No scientist ever got famous by protecting the status quo. Individual scientists will try to defend their pet theory, but there are always other scientists to pull it out from under them, if the data warrants. In the end, either the data is on your side or it isn’t.

Now we wait to see how the community reacts to this new study, and to see what new theories and observations will come of it.

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