Why Is Our Universe Expanding Faster Than Expected?

“You can think of the universe growing just like a child growing, and you can think of the cosmic microwave background as a snapshot of what the child looked like at that time. …Now, it’s the present day, the child is full grown, this is our present universe. In this case, it’s something faster (9% faster expansion) and that is becoming quite significant.”

– Nobel Laureate Adam Riess, Ph.D., a co-discoverer of dark energy

This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster PLCK G004.5-19.5 at more than 5 billion years from Earth. The light of the galaxies in the background had to travel even longer than that, making this image an extremely old window into this universe’s past. But the mystery is: why can’t astrophysicists figure out why our universe seems to be expanding faster than expected? Image: ESA/Hubble & NASA.

How fast the universe expands over time is reflected in a number known as the Hubble constant. The European Space Agency’s Planck spacecraft determined the expansion rate of the early universe by studying the cosmic background radiation, or heat, left after the “big bang” 13.8 billion years ago.

ESA: “Predictions based on the Planck data, taking into account the observed effects of dark energy, resulted in a Hubble constant of 67 kilometers per second per megaparsec. Put another way, all other things being equal, two galaxies 3.3 million light years apart — one megaparsec — should be moving away from each other at 67 kilometers per second due to the presumably constant expansion of space. The Planck data indicated a maximum upper limit of 69 kilometers per second per megaparsec.”

But Nobel Laureate Adam Riess, Ph.D., and Stefano Casertano, Ph.D., both with the Space Telescope Science Institute and Johns Hopkins University in Baltimore, Maryland, led a team of researchers who used Hubble since 2012 to refine galactic distance measurements to more accurately calculate the Hubble constant. The scientists came up with a value of 73 kilometers per second per megaparsec, a discrepancy of about 9 percent.


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