Is Cosmology, the study of the entire Universe, reaching a crisis as ever more precise measurements continue to show small but significant deviations from our models?

Back when I was in college the standard model of Cosmology consisted of a Big Bang that happened between 10-15 billion years ago. That detonation led to an expansion of the matter in the Universe that could be seen in the red shift of light coming from distant galaxies, the rate of that expansion was given the name ‘Hubble’s Constant’.

No, not that ‘Big Bang’. (Credit: Rotten Tomatoes)
This ‘Big Bang’. The evidence for this basic model of how our Universe evolved is now overwhelming. However there are still some discrepancies in the details that could be clues to new Physics. (Credit: Wikipedia)

Even as the Universe as a whole expanded locally matter clumped together due to gravity to form the galaxies and stars we see today. The model also predicted that the force of gravity between the galaxies would slow down the rate of expansion so that today Hubble’s ‘Constant’ would be smaller than it was billions of years ago.

Carl Hubble’s graph from his original 1929 paper where he announced the expansion of the Universe. The general trend of greater velocity with greater distance is easy to see but determining the exact slope of the line is very difficult. We still are having problems with that. (Credit: Universe Today)

The big Question, back when I was in college, was whether or not the force of gravity was strong enough, was there enough matter in the universe to eventually bring the expansion of the Universe to a stop billions or even trillions of years from now. If that happened the Universe would begin to contract until there was a ‘Big Crunch’. Or if there wasn’t enough matter in the Universe then it would just expand forever with all of the stars dying out as they ran out of fuel. A cold, empty Universe that was paradoxically called ‘Heat Death’ because the entire Universe would be at thermal equilibrium so that no work could be done.

Will the expansion of the Universe someday come to a stop and reverse itself into a ‘Big Crunch’ or will the expansion go on forever, maybe even accelerate. That’s the big question right now. (Credit: Astronomy Magazine)

Oh, and then there was something wrong with the way the galaxies behaved, their dynamics. They acted as if they contained more matter than we could see, so astronomers called the problem ‘Dark Matter’. The astrophysicists had a few ideas what Dark Matter could be but really had no evidence to back up their hypothesizes.

Using Newton’s law of gravity and what matter we can see in other galaxies we can calculate what their rotation curves should be. Our measurements don’t match the calculations so something is wrong. ‘Dark Matter’ is the leading theory to explain the discrepancy but it also has problems. (Credit: Universe Today)

Things began to change in the late 1990s as two groups of astronomers led by Adam Riess and Saul Perlmutter tried to answer the question of whether the expansion of the Universe was slowing fast enough to come to a stop. What they found was that the expansion wasn’t slowing at all, it was accelerating.

Along with Brian Schmidt, Saul Perlmutter and Adam Riess recieved the 2011 Nobel Prize in Physics for their discovery of the acceleration of the Universe’s expansion. (Credit: Wired)

Riess and Perlmutter used observations of Type 1 supernovas to make their measurements, see my post of 18 January 2020. Type 1 supernova occur when a white dwarf star steals matter from a nearby companion star. Eventually the white dwarf steals too much matter and explodes as a Type 1. Since all Type 1 supernova happen at the same mass our theories predict that the supernova explosions should all have the same total amount of energy and can be used to measure the distances to other galaxies. That is, if all Type 1’s are the same absolute brightness then if one Type 1 supernova looks brighter it must be closer, if another looks dimmer it must be further away.  Whatever it was that was that was pushing the galaxies apart was given the name ‘Dark Energy’ in correspondence with Dark Matter although it is really more of a pressure than an energy.

Type 1 and Type 2 Supernova are very different animals. The interesting thing about Type 1’s are that they all release about the same amount of energy so they can be used as ‘Standard Candles’ to measure distance. (Credit: Lifeng.lamost.org)

Another, more technical problem also came out of the work of Riess and Perlmutter, the value for Hubble’s Constant that they measured over the last few billion years differed slightly from the value obtained by the astrophysicists who studied the Cosmic Microwave Background (CMB), the leftover radiation from the era of the Big Bang itself.

The Cosmic Microwave Background as measured by the Planck satellite. This is our Universe’s baby picture and as such it tells us a lot about what our ‘adult’ Universe should look like. (Credit: European Space Agency)

Now a group of astronomers led by two former students of Riess, Dillion Brout of Harvard’s Center for Astrophysics along with Dan Scolnic of the Department of Physics at Duke University have published a greatly expanded data set of over 1500 Type 1 supernova observations, ten times as many as Riess and Perlmutter gathered. This study has been given the name Pantheon+ and has produced a value for Hubble’s constant over the last 10 billion years of 73.4 kilometers per second per megaparsec with an uncertainly of only 1.3%. This value is significantly larger than the value obtained from the CMB for the early Universe 13 billion years ago.

Compare this graph of the Pantheon+ supernovas with Hubble’s graph above. We’ve learned a lot in the last 90 years but there are still details to be worked out. (Credit: ResearchGate)

These measurements give us the most precise account yet of the effect that Dark Energy has had on the evolution of the Universe. It also solidifies the discrepancy between the measurements of Hubble’s constant using Type 1 supernova and those made using the CMB to a better than one in one million chance of being caused by statistical error.

In physics having enough data to declare 3 Sigma confidence, 99.7%, is considered to be ‘Evidence’ while 5 Sigma, 99.9767% is needed to declare a ‘Discovery’. (Credit: Medium)

So what is going on here?  What is causing our models and measurements to differ? Well the simplest answer would be that ‘Dark Energy’ has not been a constant effect throughout the history of the Universe, it’s dynamic, it changes and the results of Pantheon+ can give us some clues as to how it changes with time.

Dynamic simply means changing. For Cosmologists the question of whether ‘Dark Energy’ is dynamic, and if so how it changes, is the Big Question. (Credit: Adam the Automator)

The other possibility is that we’re seeing the first evidence of some completely unknown factor effecting Dark Energy. As you can imagine cosmologists are hoping to avoid that possibility. After all, currently they have no idea what Dark Energy is or if it changes. To assume there’s a yet completely unknown factor effecting Dark Energy would just square the problems we have now.

And then there’s the Dark Matter that astrophysicists first proposed before Dark Energy but which they still have no clear idea of what it is. Dark Matter was supposed to account for why galaxies, like our own Milky Way, are observed to spin faster than they should based on the matter we can see and Newton’s laws of gravity.

As you can see we’ve got a lot of ideas about what Dark Matter could be, and little evidence as to just what it is! (Credit: Physics-APS.org)

Dark Matter therefore was predicted to be some sort of heavy sub-atomic particle that did not react with the electromagnetic field, that is light, and that therefore we could not see. Physicists have been searching for that exotic particle, called a Weakly Interacting Massive Particle or WIMP, since the 1980s and have so far completely failed.

Many experiments, such as Xenon 1T shown here, have tried to discover the elusive Dark Matter particle, none have succeeded so far. (Credit: CERN)

In fact a growing minority of physicists are ready to give up on the whole idea of Dark matter and instead propose that there is something wrong with Newton’s laws of gravity. There are currently many ideas floating around as to how Newton might be wrong and these theories have been given the generic name of MOdified Newtonian Dynamics or MOND.

Modified Newtonian Dynamics or MOND seems to work theoretically but there’s no physical justification for the modifications. It’s all just math that seems to fit the data with no reason as to why! (Credit: Slide Player)

Now a new study of Open Star Clusters in our Milky Way has provided evidence backing some of those MOND theories. The paper comes from researchers at the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn in Switzerland.

The Pleiades is an open star cluster that’s easy to see with the naked eye. Known as the seven sisters there are actually hundreds of stars in a family that were born together just a few million years ago. (Credit: Wikipedia)

Open star clusters are the maternity wards of galaxies where gas clouds contract under gravity to give birth to stars. The best-known example of these open clusters are the Pleiades but many are known throughout the Milky Way and neighboring galaxies. After the stellar nursery has given birth to all the stars it can the grouping stays together for a few tens of millions of years as it orbits around the center of the galaxy. Eventually however tidal forces from the billions of other stars in the galaxy cause the stars in an open cluster to drift away, spreading across the galaxy. In fact our own Sun must have once been a member of such a cluster only to have drifted away billions of years ago.

Our Sun follows its own orbit around the center of the Milky Way but once it must have been a part of an open cluster. (Credit: NBC News)

And just as here on earth we have two tides, one rising as the Moon is high in the sky and the other 12 hours later, the tides of the galaxy will pull the stars in an open cluster in a forward direction, relative to its motion around the galactic center, and in a backward direction.

We all know that the tides are caused by the gravity of the Moon and a bit by the Sun but actually every object in the Universe has a gravity that causes a tidal force. (Credit: Time and Date)

Now Newton’s laws predict that the two tides will be of equal strength, with an equal number of stars leaving in each direction. Certain versions of MOND however predict that the forward tide should be just about twice as strong as the backward so that twice as many stars should drift away in that direction.

Newton would predict that the stars leaving an open cluster would do so equally from the front and back but the astrophysicists at the University of Bonn are seeing twice as many leave by the front. Could Newton be wrong? (Credit: Phys.org)

Needless to say trying to determine just which stars that are near an open cluster were actually once members of that cluster is no easy chore but the team from the University of Bonn succeeded with five open clusters and their results, published in the Monthly Notices of the Royal Astronomical Society strongly indicate that some variety of MOND is at work here.

So astronomy and astrophysics today have a couple of really big problems to be solved. Wouldn’t it be interesting if the solution to one problem is also the solution to the other? I mean, what if MOND is that extra factor effecting Dark Energy? We’ll just have to wait and see.