Scientists know the universe is expanding. It’s been expanding ever since the Big Bang. But it appears to be expanding faster than scientists expected – 5 to 9% faster.
A team of scientists led by Adam Riess (Space Telescope Science Institute and Johns Hopkins University) used NASA’s Hubble Space Telescope and new techniques to figure out the current expansion rate of the universe. There’s always some uncertainty with these types of measurements, but the new techniques reduced the uncertainty to just 2.4%.
How do you measure the universe’s expansion rate?
Reiss’ team kept their eyes peeled for galaxies containing Cepheid stars and Type Ia supernovae. Cepheid stars are perfect for measuring distances because they pulsate at rates that equal their true brightness. Type Ia supernovae are similar, but can be seen from much farther away.
The team found 2,400 Cepheid stars in 19 galaxies. After comparing the brightness of these stars and supernovae, they were able to measure their true brightness accurately. Armed with these results, scientists calculated distances to about 300 Type Ia supernovae in distant galaxies.
Then, scientists took those distances and compared them with the expansion of space measured by the stretching of light from galaxies moving away. These two values give us the Hubble constant, or how fast the universe expands with time. And that equals 45.5 miles per second per megaparsec (a megaparsec equals 3.26 million light-years).
Here’s an illustration showing the steps Reiss’ team used.
The new Hubble constant result does have scientists scratching their heads, though. It doesn’t match up with the expansion rate predicted for the inverse from its trajectory seen just after the Big Bang.
NASA’s Wilkinson Microwave Anisotrophy Probe shows a prediction 5% smaller than the Hubble Constant. ESA’s Planck satellite mission shows an even larger discrepancy at 9%.
Reiss talks about this in a statement.
“If we know the initial amounts of stuff in the universe, such as dark energy and dark matter, and we have the physics correct, then you can go from a measurement at the time shortly after the big bang and use that understanding to predict how fast the universe should be expanding today,” said Riess. “However, if this discrepancy holds up, it appears we may not have the right understanding, and it changes how big the Hubble constant should be today.”
Reiss compares it to building a bridge starting on both sides. “You start at two ends, and you expect to meet in the middle if all of your drawings are right and your measurements are right,” says Reiss. “But now the ends are not quite meeting in the middle and we want to know why.”
So, what’s going on? Scientists have a couple of ideas for the faster expansion of the universe. One is that dark energy is pushing galaxies further away from each other with “even greater – or growing – strength.”
Another idea points the finger at “dark radiation.” These are subatomic particles in the universe’s early history that travel close to the speed of light. More energy from ‘dark radiation could be throwing off the team’s measurements.
Or, Einstein’s theory of gravity isn’t quite complete.
Lucas Macri (Texas A&M University), who worked on the study, says it best. “We know so little about the dark parts of the universe, it’s important to measure how they push and pull on space over cosmic history.”
What’s next? Besides trying to understand dark matter better, scientists continue to work on lowering the uncertainty of their findings. It sits at 2.4%. Scientists want to lower it to 1%.
Did You Know: Reiss’ team, the Supernova H0 for the Equation of State (SH0ES) team, have already reduced the uncertainty of the Hubble constant value by 76% since 2005.
The story of the universe’s expansion rate will continue. New telescopes – like the James Webb Space Telescope and the Wide Field Infrared Survey Telescope – will only give us better measurements.