# Scientists have Solved the Problem of the Expansion of the Universe

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.