Ever since the Big Bang, the universe has been expanding. However, scientists aren’t exactly sure how fast this is happening, with different approaches for measuring the expansion giving different results.
These discrepancies could mean that one or more of the methods used to calculate the expansion of the universe is wrong. Or that there’s some kind of new physics waiting to be discovered that would sort all of this out.
Determining the Expansion of the Universe
One way that astronomers estimate the expansion of the universe — which is described by a number called the “Hubble constant” — is by measuring the cosmic microwave background (CMB). This is the remnants of the first radiation that spread across the universe after the Big Bang.
According to the Big Bang theory (the theory itself, not the popular television show), the universe underwent a rapid inflation and expansion in the beginning. The CMB is basically leftover heat from this initial explosion. It falls almost uniformly on the Earth from every direction.
The CMB has helped astronomers understand how the early universe formed, including showing when the oldest stars first started shining and providing proof for the existence of dark matter and dark energy. CMB also helps scientists estimate how fast the universe should be expanding right now.
The other main way that astronomers determine the Hubble constant is by measuring the movements of stars — how far away they are, and how quickly they’re moving away from us.
Astronomer’s look for shifts in the light emitted from stars, which gives an estimate of the star’s velocity relative to Earth. Distance is determined by using several methods, including observing stars called Cepheid variables. These stars dim and brighten at regular intervals, with the pattern of the pulses related to its brightness — brighter stars pulse more slowly.
The pulse rate gives astronomers a measure of a star’s brightness, which they can compare to what we actually see. Combining these two gives an estimate of how far away the star is from Earth. By building a “cosmic distance ladder” based on information from multiple stars, astronomers can determine how fast nearby galaxies are moving away from Earth.
Disagreements in Hubble Constant
These two methods, though, have always given conflicting results, with the more recent universe expanding nine percent faster than what would be expected from measurements of the early universe. This shows up as nearby galaxies appearing to move away from each other too quickly, based on observations.
To help resolve this discrepancy, an international group of astronomers measured the Hubble constant using a third method — gravitational lensing. As light streaming to the Earth from a distant object passes a massive galaxy, the gravity of that galaxy acts as a lens that bends the light.
As a result of the light bending, astronomers on Earth see multiple images of the distant object. When the brightness of the object changes, there is a time delay in when that flicker shows up in the images. This can be used to determine the Hubble constant.
In the new study, the team of astronomers repeated this process on three quasars, bright stars at the center of certain galaxies. Their measurement of the Hubble constant matched what is found with the cosmic distance ladder method. This provides support for this estimate of the Hubble constant for the more recent universe.
However, it doesn’t explain why the universe appears to be expanding faster now than the CMB method shows for the early universe. Astronomers are now working on trying to understand whether this discrepancy represents a fundamental problem with our universe or a problem with our standard view of cosmology.