September 10, 2013
Hubble Bubble May Explain Oddities In Expansion Of The Universe
John P. Millis, PhD for redOrbit.com - Your Universe Online
For the better part of a century the astronomical community has understood that the Universe is expanding, yet the exact nature of this expansion has been difficult to quantify. One extreme instance of this is the acceleration of galaxies receding from us billions of light-years away. But even looking closer to home, calculating the rate of expansion has proven difficult.
Traditionally, scientists, measuring the recession velocity of galaxies in the nearby part of the Universe, are able to arrive at a value for the rate of expansion. Plotting the distance to these galaxies against their speeds, just as Edwin Hubble originally did about 90 years ago, gives Hubble’s Constant, the desired rate.
However, one could also measure the Cosmic Microwave Background (CMB) – the radiation released shortly after the Big Bang that has been spreading out as the Universe expands. However, the most detailed measurements of the CMB reveal a discrepancy of about 9 per cent when compared to galactic velocity methods.
This presents a puzzling problem. But now, researchers from Heidelberg University in Germany collaborating with physicists from the Netherlands have proposed that our galactic neighborhood exists within a “Hubble Bubble," instead of relying on some as-yet-unknown instrumental error with the measurements. In this theory such a bubble would be defined by parts of the Universe where the density of matter is below the average value for the Universe.
“Until now knowledge of our cosmic neighborhood has been too imprecise to determine whether or not we are in such a bubble," says Valerio Marra from Heidelberg University’s Institute for Theoretical Physics. “But let's just 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.”
Calculations of the expansion rate of the Universe based on measurements of the CMB would represent the average expansion rate of the Universe as a whole, while studies examining galactic recession velocities would be valid primarily for our little corner of the cosmos.
So far, their model is able to account for about one fourth of the discrepancy between the measurements. However, the team has already identified areas for improvement that they hope will bridge the gap even further.
“Until now we have been working with a spherical Hubble Bubble. But it's far more likely that the bubble is asymmetrical, which would explain the deviating measurements even better," explained Dr. Ignacy Sawicki, also a researcher at the Institute for Theoretical Physics.
“If the difference in data should manifest itself instead, this would be a major indicator that our former natural scientific view of the cosmos is still missing an ingredient.”
The German-Dutch team's study, titled "Cosmic Variance and the Measurement of the Local Hubble Parameter," appears in the latest issue of the journal Physical Review Letters.