If you google the term “Crisis in Cosmology” you’ll get a lot of hits.  For the last few years, a lot of scientists have been driven to ask if there’s something really wrong with our best model for the Universe.  Today I want to give you one specific way this “crisis” is playing out.

Talk to an astronomer about the state of the art in understanding the Universe and they’ll tell you we’ve entered the Precision Age of cosmology.  The data relevant to cosmic evolution have gotten so good that we know all the relevant parameters – things like the average density of matter in the Universe - down to a few decimal places.  That’s a pretty impressive achievement.

One of the most important of these cosmic parameters is what’s known as the Hubble Constant (cosmologists write it as H_o).  Modern cosmology tells us the Universe has been expanding since its beginning in the Big Bang.  The Hubble Constant tells you the speed of that expansion.  It’s also related to the age of the Universe.  Larger values of H_o mean a younger Universe. Smaller values of H_o mean an older Universe. 

Back when Edwin Hubble first discovered that all the galaxies were flying away from each other (i.e. the Universe was expanding), his crude data gave H_o = 500 (we’ll ignore the units).  This value was so large it gave an age of the Universe that was shorter than the age of the Sun or the Earth.  Oops! You can’t have a Universe that’s younger than the stuff in it. Better measurements soon gave much lower values of H_o which meant a Universe thankfully older than the Earth, Sun and Stars. Conflict resolved.

But the idea of conflicts with measured values of H_o didn’t go away.  Today it’s a conflict between different ways of measuring H_o that’s making big news.

There are basically two ways to measure the Hubble Constant.  The first is based on looking at what cosmologists call the “late” Universe.  This means looking at stuff that’s relatively recent in cosmic history like exploding stars called Supernova. This method gives a Hubble Constant of H_o = 74.03.

The other method relies on data from the “early” Universe, i.e., the cosmos right after the Big Bang.  Light emitted by matter about 300,000 years after the cosmic beginning provides astronomers with a rich source of early Universe measurements. This kind of data gives H_o = 67.40 which clearly smaller than the late Universe supernova data.

This difference (74.03 vs 67.40) might seem small to most of us, but it’s actually big enough to drive a cosmic truck through.  The conflicting results mean we can’t pin down other key cosmic properties like, for example, the age of the Universe (13.6 vs 13.7 billion years ago). That leads to other big problems in explaining the finely tuned dance of cosmic evolution that must lead to the conditions we see today (conditions that allow life like us to exist).

Worst of all, over the last few years measurements have gotten so good that the uncertainty associated with them is tiny.  So the problem can’t be with errors in instruments or methods.  The values of the Hubble Constant obtained from the two methods really are different.

Here is why that’s a problem.

Our dominant cosmological theory is something called the “Lambda Cold Dark Matter” or LCDM model (L would be the Greek letter “Lambda”).  The model assumes the Universe is mainly built of dark energy and a slow-moving (i.e cold) form of Dark Matter.  This model makes predictions that have been very, very well tested.  In other words, it works in lots of different ways.

But it can’t explain the tension between the two methods of determining the Hubble Constant.  The stubborn “Hubble Tension” between the two values means something fundamental may be fundamentally wrong with that cherished LCDM model. So should we get rid of it?   

LCDM works so well, in so many ways, that it’s not something one throws out lightly. Any change to it will have consequences that can mess up all the places where it already does work in explaining what we see in the Cosmos.  That’s what, in the end, the tension in Hubble’s Constant offers us a lesson in how science progresses when things go wrong. 

Cosmologists have a paradigm they love, and it mostly works. But along comes this problem in the form of the Hubble Tension and, as philosopher of science Thomas Kuhn pointed out, there are typical ways scientists will respond to such a problem. At first everyone thinks the problem will go away.  But then it doesn’t.  So, what should they do? They could tinker with the old theory in a way that looks jury-rigged.  Or they could abandon the old theory entirely but at enormous cost. They could also keep poking around and hope things just work themselves out. 

So what should they do?  What would you do?

Finally, it’s important to note two things.  The first is that none of this challenges the basic Big Bang idea i.e. the Universe expanded from an ultra-hot ultra-dense soup of matter and energy around 14 billion years ago.  That fact is not in question. The second point is that a science in crisis is not a bad thing.  Instead, it's exciting.  What could be cooler than to find out you need to do more and better science to figure out what’s really going on.

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PS If you have specific questions or issues you want me to address leave a comment on the website or email me at [email protected]. I’ll definitely read it and will try to respond asap.

PPS I could not get this proof-read so please excuse typos etc.

— Adam Frank 🚀