The researchers at Cosmic Dawn Center found that the measurements of velocity used for determining the expansion-rate of the Universe might not be reliable. As stated in the publication, this does not resolve the discrepancies, but rather hints at a further inconsistent in the composition of the Universe.
Measuring the expansion rate of the Universe
Now, astronomers measure the expansion of the Universe using two different techniques. One is related to the measuring connection between distance & velocity of nearby galaxies, while the opposite-stems from studying the background from the very early universe. Surprisingly, these two approaches currently find different expansion rates. If this discrepancy is real, a brand new & dramatic reinterpretation of the evolution of the Universe are going to be the consequence. However, it’s also possible that the difference in the Hubble’s constant might be from incorrect measurements. It’s difficult to measure distances in the Universe, numerous studies have focused on improving & recalibrating distance measurements. But in spite of this, over the last 4 years the disagreement hasn’t been resolved.
The velocity of the remote galaxies is straightforward to measure — approximately we thought
In the recent scientific article, the researchers from the Cosmic Dawn Center now plan to light on a related problem: the measurement of velocity. Depending on the velocity with which a foreign object moves away from us, its light shifts to redder colors. With this so-called redshift, it’s possible to find the velocity from a spectrum of a foreign galaxy. Unlike measurements of distance (until now), it had been assumed that velocities were relatively easy to find.
However, when the researchers recently examined distance & velocity measurements from more than 1000 supernovae (exploding stars) collected during the last 25 years, they found a surprising discrepancy in their results. Albert Sneppen, Masters student at Niels Bohr Institute explains: “We’ve always believed that measuring velocities was fairly straightforward & precise but it seems that we are literally handling two sorts of redshifts.”
The first type, finding the velocity with which the host-galaxy moves away from us, is taken into account the most reliable. The another sort of redshift measures instead the velocity of matter ejected from the exploding star inside the galaxy. Or more precisely, the matter from the supernova moving towards us with a couple of percent of the velocity of light (illustration 1). After compensating for this extra movement the redshift and velocity of the host galaxy are often determined. But this compensation requires a particular model for the explosion. The researchers were ready to determine that the results from these two different techniques result into two different expansion histories for the Universe and thus 2 different compositions also .
Are things “broken in a stimulating way?”
So, does this mean that the measurements of the early Universe & newer measurements are ultimately an issue of imprecise measurements of velocity? Probably not, says Bidisha Sen, one the authors of the article. “Even if we only use the more reliable redshifts, the supernova measurements not only still continue to disagree with the Hubble’s constant measured from the early Universe, they also hint at a more general discrepancy regarding the composition of the Universe” she says.
Associate professor at the Niels Bohr Institute, Charles Steinhardt is intrigued by these new results. “If we are literally handling two disagreements, it means our current model would be “broken in a stimulating way” he says. “In order to solve two problems, one regarding the composition of the Universe and one regarding the expansion rate of the Universe, rather different physical explanations are required than if we only want to elucidate one discrepancy in the expansion rate.”
The Scientific work continues at the Nordic astronomical telescope
With the Nordic astronomical telescope in Gran Canaria, the researchers are now acquiring new redshifts from the host galaxies. When they compare these results with the supernova based redshifts, they’ll be able to see if the 2 techniques remain different. “We have learned that these sensitive measurements require precise measurements of velocity and these will be attainable with fresh observations” Steinhardt explains.