The Hubble constant describes how fast the universe is expanding, but our
measurements won’t line up, which may mean our standard model of the universe
is wrong
The expansion of the universe is accelerating, but we don’t know how
quickly. With new observations, this issue has only got more severe, and now
some astronomers are saying that it is officially a real problem – not one
caused by uncertainties in the measurements.
There are two main ways we measure the Hubble constant, which describes the
expansion of the universe. The first is to examine the cosmic microwave
background – a relic of the first light to shine through the universe after
the big bang – and use our standard model of cosmology to calculate what the
expansion rate should be like today. This puts the rate of acceleration at
about 67 kilometres per second per megaparsec.
The other method, called the local method or the distance ladder, involves
measuring the distances to stars called cepheids and then using those
distances to extrapolate to supernovae in other galaxies. These distances
allow us to calculate the Hubble constant, which the latest measurements
from Adam Riess at Johns Hopkins University in Maryland and his colleagues
have put at about 73 kilometres per second per megaparsec.
For decades, it has been plausible that these two methods would eventually
converge on a single true value of the Hubble constant. Now, Riess and his
team say that is extraordinarily unlikely – which would mean that something
is wrong with our standard model of the universe.
Even after analysing the data in many different ways and including results
from other teams, “it’s really hard for us to get below about 72.5 or above
about 73.5,” says Riess. The disagreement between the two calculations is
known as the Hubble tension.
By his team’s calculations, the two methods of measurement disagree with one
another at a statistical level referred to as “5 sigma”, generally
considered a gold standard in physics for demonstrating that measurements
are a true discovery and not a statistical fluke. This means there is only
about a 1 in 3.5 million chance that the Hubble tension is just a fluke.
However, other astronomers have pointed out that even a 5-sigma discrepancy
doesn’t rule out the possibility of errors or systematic uncertainty in our
measurements of stars. “It doesn’t matter how many sigma away you are, it’s
whether you have determined all of the potential errors out there that had
led to that place,” says Barry Madore at the Carnegie Institution for
Science in California.
While the measurements may point towards the Hubble tension being a real
problem, we cannot know for certain until it is confirmed by several methods
of measurement, says Madore. Thankfully, the James Webb Space Telescope
should be able to help with that and researchers are also working on other
methods, such as using gravitational waves.
If the tension is shown to exist, the specifics of the new physics we would
need to explain it are still up in the air. “We’re very clear that we don’t
know what the source of the Hubble tension is,” says Riess. “There have been
some intriguing ideas, but none of them are a perfect fit yet.”
Reference:
arxiv.org/abs/2112.04510
Tags:
Space & Astrophysics