We Might Be Totally Wrong About the Universe's Expansion


Researchers have obtained the most accurate measurement yet of how quickly the universe is expanding — and it's not what they were expecting to find.

The universe is expanding about 8% faster than predicted, according to a new study published on the preprint server arXiv.

If the results are confirmed, it could transform our understanding of dark matter and dark energy, and completely change our understanding of cosmology itself. 

"I think that there is something in the standard cosmological model that we don't understand," Adam Riess, co-discoverer of dark energy and lead author on the new paper, told Nature.

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A cosmological mystery: Cosmologists believe dark matter is an invisible material that makes up 27% of the universe, while visible matter only makes up 5%. The fact that we can measure dark matter's gravitational pull suggests that it exists.

Dark energy makes up the other 68% of the universe. It's an unknown force that cosmologists think is accelerating the expansion of the universe. 

Cosmologists think dark matter puts the breaks on expansion, while dark energy accelerates it.

A lot of what we know about dark matter, dark energy and the universe's expansion is based on our observations of radiation left over from the Big Bang called the Cosmic Microwave Background, or CMB.

Recently, scientists put together the most detailed maps of the CMB we've ever had. Think of the maps as the baby pictures of our universe. Based on those baby pictures, scientists can predict how our universe expanded and evolved — and what its current rate of expansion should be. 

The problem? The latest measurements in this study don't match those predictions. 


Reiss and his team measured the expansion of the universe by studying certain types of celestial objects, like stars and supernovae, known as "standard candles." Standard candles are thought to all be the same level of brightness, so astronomers can use them as distance markers to measure how fast the universe is expanding away from us. If this new measurement is accurate — and our maps of the CMB are also accurate — then something about our fundamental understanding of the universe is wrong.

The researchers have possible explanations. Dark matter might have some property that we don't know about that's driving expansion. Another possible explanation for the discrepancy is that dark energy has simply gotten stronger over time, and sped up the universe's expansion rate. 

Or, it might be that our way of measuring expansion isn't very accurate. Cosmologists rely on standard candles to tell them how quickly objects are moving away from us, but there's a growing body of evidence that these candles might not be so standard after all, and therefore not an accurate measurement of the rate of expansion.

We'll have to wait and see if the new study holds up. If it does, cosmology as we know it may need a big overhaul.

h/t Nature