[D66] The growing crisis in cosmology

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The growing crisis in cosmology
By
Matthew Francis
theweek.com
4 min
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How rapidly is the universe expanding?

Since Edwin Hubble first discovered in 1929 that galaxies are getting 
farther apart over time, allowing scientists to trace the evolution of 
the universe back to an initial Big Bang, astronomers have struggled to 
measure the exact rate of this expansion. In particular, astronomers 
want to determine a number called the Hubble parameter, a measurement of 
how fast the cosmos is expanding as we speak. The Hubble parameter tells 
us the age of the universe, so measuring it was a major goal for many 
astronomers in the latter half of the 20th century.

The problem, however, is that measuring the Hubble parameter is, perhaps 
unsurprisingly, quite difficult. There are multiple methods for doing 
so, and modern observatories are coming up with different numbers 
depending on which method they use. It seems the number obtained based 
on the appearance of the universe shortly after the Big Bang is 
significantly smaller than the number obtained when looking at 
measurements involving objects closer by.

The early universe Hubble parameter, derived from observations by the 
European Space Agency's Planck satellite, tells us the universe is about 
13.8 billion years old. Meanwhile, the local cosmos measurements might 
yield an age nearly a billion years younger. If that smaller age is 
correct, it throws off the entire timeline of cosmic history, and could 
mess up our understanding of when and how various major events happened 
in the evolution of the universe.

To be clear: This discrepancy isn't so huge that the Big Bang theory is 
in trouble, or that we have to rethink everything we know about the 
cosmos. But the discrepancy is large enough that some cosmologists — 
scientists studying the history and makeup of the universe as a whole — 
are suggesting the field is in crisis.

Adam Riess, the Johns Hopkins cosmologist who shared the 2011 Nobel 
Prize in physics, has argued strongly that we can't ignore the 
discrepancy, because it keeps appearing, over and over again, in too 
many independent local cosmos observations to be a fluke. "If the 
universe fails this crucial end-to-end test (it surely hasn't yet 
passed), what might this tell us?" Riess wrote in Nature. "It is 
tempting to think we may be seeing evidence of some 'new physics' in the 
cosmos."

That could be the case, but "I would say it's at least as likely that we 
still don't understand what all the subtleties in these measurements 
are," says cosmologist Arthur Kosowsky of the University of Pittsburgh 
(who was the author's Ph.D. advisor), "and eventually they'll converge 
to a single value [for the Hubble parameter]."

Indeed, the discrepancy could come down to little more than hidden 
biases in the measurements. To use an analogy, if you have an air rifle 
that pulls very slightly to the right as you shoot at a target, all your 
shots may be clustered around a single point, but that point will be to 
the right of the actual bullseye. The rifle introduces a systematic 
error to your normally good aim, right?

But the analogy is imperfect because in cosmology, we don't know where 
the "bullseye" is: The precise value of the Hubble parameter can't be 
calculated independently of measurements. Complicating matters further, 
none of the observations measure the Hubble parameter directly. Instead, 
they link different observable phenomena to the rate of cosmic 
expansion. The trick is to use multiple independent measurements as a 
check on one another, hoping that any systematic effects can be spotted 
in the process.

For example, the Planck satellite's early-universe observations — those 
that tell us the universe is expanding more slowly — are based on 
something called the cosmic microwave background (CMB), which is light 
left over from about 400,000 years after the Big Bang. However, we 
aren't seeing this light as it was way back then; we're seeing it after 
it's passed through clusters of galaxies and entered the Milky Way, with 
extra light from other sources added in. To get anything useful out of 
CMB data, astronomers have to subtract everything that isn't primordial; 
while they're very good at that, there might still be room for 
systematic bias in the way the subtraction is done.

Measurements based on closer objects — which yield a slightly younger 
age — are based on type Ia supernovas, which are the explosions of 
super-dense objects called white dwarfs; the pulsations of very large 
stars; and the gravitational distortion of light as it passes by 
galaxies. Each type of measurement has its own set of systematic biases 
that must be corrected. It's also worth noting that while their Hubble 
parameter values are close to each other, they aren't in precise 
agreement either. In other words, all of these observations are complex 
enough that their results aren't completely settled.

New types of observation are underway, which might help identify where 
bias exists in Hubble parameter measurements. Removing those biases 
would bring the Hubble numbers into agreement.

But what if the numbers are right? That could be indicative of some 
previously unknown phenomenon in the very early universe, from the epoch 
before the first atoms formed. Perhaps dark energy, which we know is 
driving cosmic expansion to accelerate in the modern era, played a role 
far earlier than most cosmologists think. Maybe there are extra 
particles that were important when the universe was smaller and denser, 
but whose influence was diluted over billions of years.

None of these possibilities are perfect, but whatever the right answer, 
it's a big deal.

"I don't have a crystal ball better than anyone else, but I think that 
these measurements are just really hard," Kosowsky says. However, his 
caution doesn't mean he wouldn't be happy to be wrong. "I'm rooting for 
it to come out the other way, that this is actually showing us something 
exciting about cosmology."