'Hubble Bubble' Explains Differing Expansion Rates of the Universe

The "Hubble Bubble" may just help explain the differing measurements for the expansion rate of the universe. Scientists have created a theoretical model that places the Milky Way inside this type of cosmic bubble, revealing that it can explain some of the deviations between previous measurements and the latest ones.

The universe has been expanding since the Big Bang, causing galaxies beyond our Local Group to appear to be receding from us as it continues to push outward. The actual speed of this expansion is known as the Hubble constant. It's a crucial part of calculating the basic properties of the universe, such as its age.

Yet actually determining this constant is another matter. Researchers usually use two conventional methods. However, the results from these methods don't agree, which means that the constant has been a source of long-standing debate in the scientific community.

One of the techniques to determine the Hubble constant is based on measuring the cosmic microwave background radiation in space. This radiation was released about 400,000 years after the Big Bang and pervades the entire universe. The other technique is deriving the constant from the movement of galaxies near the Milky Way, motion that's largely due to the expansion of the universe.

"When you compare the results from the two methods, there is a deviation of about 9 percent," said Valerio Marra, one of the researchers, in a news release.

That's why the researchers decided to investigate this phenomenon a bit further. They believed that the difference could be based on a physical effect--the Hubble Bubble. The bubble describes regions of the universe where the density of matter falls below the cosmic average.

"Until now, knowledge of our cosmic neighborhood has been too imprecise to determine whether or not we are in such a bubble," said Marra in a news release. "But let's assume for a moment that our Milky Way is located in a Hubble Bubble. Matter outside the bubble would then attract nearby galaxies so strongly that they would move more quickly than average. In this case we would measure a higher Hubble constant that would apply to our cosmic neighborhood, but not to the universe as a whole."

This bubble could help explain the conflicting measurement results. In fact, so far the scientists have been able to account for about one-fourth of the deviation between the Hubble constants. The researchers believe that with a more detailed analysis, this discrepancy could be reduced even more. This, in turn, could help researchers better understand what the true constant is which could help them learn more about our universe.

The findings are published in the journal Physical Review Letters.