How fast is
our universe expanding? To answer this question, scientists used two different
methods and found two answers that are slightly different from each other and
that's the source of 'Hubble tension.' Astrophysicists have been divided into
two groups, one which thinks this difference in the answers is significant and
we need new physics to explain it. Others attribute it to the difference in the
methods. A recent study now says that the difference may not be that different
at all.
Hubble
constant is the rate at which the universe expands. Knowing this number can
help us understand how old is our universe and how it has evolved. To determine
this number, scientists used two approaches.
In one, they
looked at the faint light left after the Big Bang. Called the cosmic microwave
background, this light was measure in space as well as on the ground using
telescopes. The observations were fed into the 'standard model' of the early
universe and use to estimate the Hubble constant today. The answer is 67.4
kilometers per second per megaparsec (km/s/Mpc)
The second
method is to look at stars in a nearby universe and measure how fast are they
moving away from us. In 2001, Wendy Freedman and her team at the University of
Chicago used the Hubble Space Telescope to look at stars called Cepheids. They
found the Hubble constant to be 72 km/s/Mpc.
Freedman and
her team continued to look at Cepheids over the years but in 2019 decided to
cross-check their method by looking at stars called 'red giants'. These are
very large and luminous stars that reach peak brightness and then fade rapidly.
By measuring the actual peak brightness, scientists can measure distances to
their host galaxies. But the measurements need to be accurate. So, Freedman and
her team used four different measurement methods for different stars and
galaxies and found them to be accurate within one percent error. They then used
red giants to estimate the Hubble constant and found its value to be 69.8
km/s/Mpc, much closer to the value derived by observing the cosmic microwave
background.
Explaining
the cause for differences in two values her team derived, Freedman said,
"The Cepheid stars have always been a little noisier and a little more
complicated to fully understand; they are young stars in the active
star-forming regions of galaxies, and that means there's potential for things
like dust or contamination from other stars to throw off your
measurements."
With the
launch of the James Webb Space Telescope, Freedman is confident that with
higher resolution and sensitivity, the data will improve in the near future.
"There
is still some room for new physics, but even if there isn't, it would show that
the standard model we have is basically correct, which is also a profound
conclusion," Freedman added. The study will be published in Astrophysical
Journal.
No comments