Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of “Ask a Spaceman” and “Space Radio,” and writer 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 growth price 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 so much (and it is not, contemplating the appreciable feats of science wanted to make these measurements within the first place), the uncertainties in these measurements have been solely about 2%.
A ten% distinction with a 2% uncertainty is statistically vital 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.
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 couple of base assumptions, identical to every other mannequin in science. The mannequin assumes that general relativity is appropriate 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’s 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 is one other substance, known as dark energy (that is the “Lambda”), that maintains a relentless density because the universe expands.
Once these assumptions are set (and they’re, primarily based 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 little thing else in regards to the universe, as much as and together with the present-day growth price.
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 previous. The CMB is beneficial as a result of it is large, simple to measure and easy to grasp.
Armed with measurements of the CMB, like these obtained from the European Space Agency’s Planck satellite mission, you’ll be able to fill within the unknowns within the LCDM mannequin and have a very good grip on principally your complete historical past of the cosmos.
Ladders to the celebrities
So right here comes the strain. 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 in addition from so-called baryon acoustic oscillations — refined shifts in galaxy positions left over from when big sound waves crashed round within the early universe — and the abundance of sunshine parts.
No matter what mixture of early-universe measurements you make to fill within the LCDM mannequin, you find yourself predicting a price of the Hubble fixed someplace round 68 km/s/Mpc.
OK, nice. Job over, proper? Not so quick.
You can even attempt to immediately measure the Hubble fixed. To do that, you might want to measure the distances and speeds of a bunch of objects. There are a number of choices, together with Type Ia supernovas, galaxy properties, Mira variables and sure sorts of red giant stars.
With the exception of the red-giant technique, all the native measurements of the Hubble fixed reveal a a lot greater quantity — extra like 74 km/s/Mpc.
Interestingly or frustratingly, the red-giant technique gives a number right in the middle of the 2 extremes — therefore, the disaster.
No method out
We have two utterly alternative ways of approaching the Hubble fixed. Both are effectively examined, effectively studied and effectively understood. The LCDM mannequin is wildly profitable in describing and predicting a number 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 your complete 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, massive scale versus small scale — irrespective of the way you slice it, these two views on the universe ought to agree, however they do not. We ought to have a standard, agreed-upon worth for the Hubble fixed, however we do not.
Cosmologists are so on this “crisis” as a result of it is the primary fascinating factor to occur in cosmology because the discovery of darkish power 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 within the period of the CMB. Some name for darkish power doing one thing bizarre within the current 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 an answer.
I personally consider 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 individual query on Twitter utilizing #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.