Why is there a ‘crisis’ in cosmology?

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of “Ask a Spaceman” and “Space Radio,” and creator of “How to Die in Space.” Sutter contributed this text to Space.com’s Expert Voices: Op-Ed & Insights.

Beginning in 2014, measurements of the Hubble constant — the present-day enlargement charge of the universe — started to disagree. Measurements taken from the distant universe have been about 10% off from measurements taken from the close by universe. While that does not sound like a lot (and it is not, contemplating the appreciable feats of science wanted to make these measurements in the primary place), the uncertainties in these measurements have been solely about 2%.

A ten% distinction with a 2% uncertainty is statistically important and one thing price investigating. Since 2014, there have been over 300 proposals for options to this “crisis in cosmology.” None of those proposals is universally agreed upon by cosmologists, and as measurements proceed, the disaster simply retains getting worse.

Related: Is there really a ‘crisis’ in cosmology?

The LCDM mannequin

What’s at stake right here is our fashionable understanding of the historical past of the universe, as encapsulated by the so-called Lambda-CDM mannequin, usually abbreviated as LCDM. This mannequin makes a few base assumptions, identical to some other mannequin in science. The mannequin assumes that general relativity is right at cosmological scales and that our universe is homogenous and isotropic (the identical in all instructions). It assumes that our universe is geometrically flat and that there is some entity, known as dark matter, that does not usually work together with regular matter (that is the “CDM” half, for “cold dark matter”). And it assumes that there’s one other substance, known as dark energy (that is the “Lambda”), that maintains a fixed density because the universe expands.

Once these assumptions are set (and they’re, based mostly on quite a few previous observations), the LCDM has solely six free parameters. You have to make numerous cosmological measurements to get these numbers, however when you do, the mannequin can predict every part else in regards to the universe, as much as and together with the present-day enlargement charge.

One of the perfect locations to pin down the values of the six free parameters is the cosmic microwave background (CMB), which is the sunshine left over from when the universe was solely 380,000 years outdated. The CMB is helpful as a result of it is massive, straightforward to measure and easy to know.

Armed with measurements of the CMB, like these obtained from the European Space Agency’s Planck satellite tv for pc mission, you’ll be able to fill in the unknowns in the LCDM mannequin and have a actually good grip on mainly all the historical past of the cosmos.

Ladders to the stars

So right here comes the stress. Measurements of the early universe give us a great deal of details about the free parameters of the LCDM mannequin. Those measurements come from not solely the CMB but additionally from so-called baryon acoustic oscillations — delicate shifts in galaxy positions left over from when big sound waves crashed round in the early universe — and the abundance of sunshine parts.

No matter what mixture of early-universe measurements you make to fill in the LCDM mannequin, you find yourself predicting a worth of the Hubble fixed someplace round 68 km/s/Mpc.

OK, nice. Job over, proper? Not so quick.

You also can attempt to immediately measure the Hubble fixed. To do that, you’ll want to measure the distances and speeds of a bunch of objects. There are numerous choices, together with Type Ia supernovas, galaxy properties, Mira variables and sure sorts of red giant stars.

With the exception of the red-giant methodology, the entire native measurements of the Hubble fixed reveal a a lot greater quantity — extra like 74 km/s/Mpc.

Interestingly or frustratingly, the red-giant methodology gives a number right in the middle of the 2 extremes — therefore, the disaster.

No way out

We have two utterly other ways of approaching the Hubble fixed. Both are nicely examined, nicely studied and nicely understood. The LCDM mannequin is wildly profitable in describing and predicting a host of cosmological observations, so no one is in that nice of a rush to toss it out. The measurements manufactured from the CMB are beautiful — by far essentially the most correct measurements taken in all the historical past of astronomy.

On the opposite hand, the supernova measurements are additionally legit. And totally different probes of the Hubble fixed are giving comparable solutions.

Early universe versus late universe, international versus native, giant scale versus small scale — regardless of the way you slice it, these two views on the universe ought to agree, however they do not. We ought to have a frequent, agreed-upon worth for the Hubble fixed, however we do not.

Cosmologists are so in this “crisis” as a result of it is the primary attention-grabbing factor to occur in cosmology for the reason that discovery of dark energy over 20 years in the past. When measurements disagree, it is nature whispering to us. There’s room right here, and maybe a possibility, to study extra in regards to the cosmos.

To date, there have been over 300 proposals for options to the cosmology disaster. Some name for extra physics in the period of the CMB. Some name for dark energy doing one thing bizarre in the latest previous. Some alter physics at a fundamental level, messing up our observations of supernovas.

However, no single proposal can clarify the wealth of cosmological proof, and we’re nowhere close to a consensus for a answer.

I personally imagine that “if it’s interesting, it’s probably wrong.” The most boring rationalization for the disaster is that we’re misunderstanding one thing about measuring the Hubble fixed at native scales.

But solely time will inform.

Learn extra by listening to the “Ask A Spaceman” podcast, accessible on iTunes and askaspaceman.com. Ask your personal query on Twitter utilizing #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter

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