Crisis In Cosmology Gets Worse

Cosmologists are perplexed. They believe they have a good understanding of the origins of the universe and how it has evolved since the beginning. However, two measurements of the speed at which the universe is currently expanding disagree and that could be the first signs that they will have to make significant changes to their understanding of the cosmos. 

A recent measurement has deepened the controversy. The dispute is basically simple. Scientists used two ways to determine the current expansion speed of the universe. The first involves making measurements of the conditions of the universe when it began and then using well accepted theory to predict today’s expansion rate. The second is to simply measure the rate today. If everything hangs together, the two numbers should agree. But they don’t.

Predicting the current expansion rate of the universe using ancient data is complicated.  The ancient data includes the cosmic microwave background, which is a radio signal that is essentially the cooled fireball of the Big Bang. Other data includes the pattern of how galaxies gathered over time. For instance, did galaxies tend to cluster together, leaving voids? Or were they dispersed uniformly? And how did those patterns change over billions of years?

Astronomers take all of those measurements and more, and combine them with a sophisticated theory of the evolution of the universe to predict a rate at which the universe should currently be expanding. They predict that the expansion rate of the universe should be 67.4 ± 0.5 kilometers per second per megaparsec distance. A megaparsec is 3.26 million light years. This means that a galaxy a megaparsec away from Earth should be moving away at 67.4 km/s, while a galaxy two megaparsecs away should be moving away from us at a speed of 134.8 km/s. 

However, astronomers can also directly measure the expansion rate, simply by looking at galaxies within a few million light years and they find a much larger expansion rate. The directly measured expansion rate is 73.2 ± 1.3 km/s per megaparsec.

The crux of the disagreement is that 67.4 ± 0.5 and 73.2 ± 1.3 disagree.

So, what can explain this disagreement? Well, each method has assumptions and limitations that should be revisited. For instance, when astronomers measure the expansion rate of the universe today, they look at individual galaxies and determine each galaxy’s speed and distance. The speed is easy to determine. Astronomers use what is called the Doppler effect. This effect makes galaxies moving away from the Earth look redder than they would if they were stationary. Furthermore, the faster they are moving, the redder they appear. Color is easy to measure, so we know each galaxy’s speed very well.

But a galaxy’s distance is much more difficult. In fact, it has taken over a century to work out a method for determining cosmic distances. For short distances – say 10 – 100 light years, astronomers use triangulation. They look at the location of a nearby star on one night and then again six months later. If the star is relatively nearby, its location will appear different compared to more distant stars. And by using the diameter of the Earth’s orbit, along with the very small different positions the nearby star appears, astronomers can work out the star’s distance.

For more distant stars or galaxies, scientists use stars or supernovae of known intrinsic brightness. By comparing the object’s intrinsic brightness with the observed brightness in our telescopes, we can work out its distance. For objects that are thousands to a few million light years away, astronomers use a type of stars call Cepheid variables, which vary in brightness. The intrinsic brightness of the star is related to the amount of time between consecutive bright periods. For objects that are millions to billions of light years away, astronomers use a class of supernovae called Type Ia. These are stars that explode with an intrinsic brightness that is something scientists can determine.

The entire spectrum of distances is tied together. Astronomers use triangulation on nearby Cepheid variable stars to determine their intrinsic brightness. They then look at Cepheid variable stars in nearby galaxies in which Type Ia supernovae have occurred to determine the supernovae’s intrinsic brightness. This interconnectivity is called the cosmic ladder, where each distance scale is connected to the one below it.

So, this means that everything is tied to getting a firm grasp on triangulating the distance to nearby stars. If that’s wrong, all other distance scales are also wrong.

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n 2013, the European Space Agency launched the Gaia mission. Gaia is a space platform that is able to measure the location of nearby stars with unprecedented precision. The spacecraft has many missions, for example, making a precise 3D map of the nearby parts of the Milky Way galaxy. However, a group of astronomers have also used the data set to very precisely determine the distances to nearby Cepheid variable stars. This, in turn, results in a precise determination of the current expansion rate of the universe, specifically 73.2 ± 1.3 km/s per megaparsec, with a precision of 1.8%. This is to be contrasted with an earlier estimate of 74.03 ± 1.42 km/s per megaparsec. The precision of this earlier estimate was 1.9%. Furthermore, the researches expect that the Gaia data will allow them to eventually achieve a precision of 1%.

So, it appears that there is a real and significant difference between the direct measurement of the current expansion rate of the universe and a prediction using data from billions of years ago.

Turning to the prediction using data from the dawn of the cosmos, what sorts of weaknesses does that effort have? Well, for one, it assumes that our accepted theory of the evolution of the universe is correct. However, it is entirely possible that this theory doesn’t include unknown phenomena. Suppose, for example, that during the first million or so years after the Big Bang there was a period where gravity didn’t slow down the expansion of the universe, but briefly sped it up. Metaphorically, if the expansion rate of the universe was a car, perhaps something stepped on the accelerator for a short period of time.

The idea of a brief period of early accelerated expansion would require some unknown physics and would require a modification of the theory of the evolution of the universe. This is by no means universally accepted, but it is a possible solution of the disagreement between two methods of measuring the current expansion rate of the universe.

And, of course, other scientists are trying to find other ways of measuring this rate. For instance, while the traditional way of setting the first rung of the cosmic ladder is using Cepheid variable stars, other astronomers are attempting to use other approaches, for example using RR Lyrae stars, tip-of-the-red-giant-branch stars, and so-called carbon stars.

We don’t know how this cosmic discrepancy will be resolved, but it appears to be a real cause for concern. On the mundane side, it could be that there is a conceptual error in one or more of the current analyses. On the exciting side, it could be that there is more to learn about the evolution history of the cosmos. We’ll just have to wait for the answer.