The universe is increasing sooner than predicted by common fashions in cosmology. Credit score: NASA
Astronomers have recognized for many years that the universe is increasing. After they use telescopes to watch faraway galaxies, they see that these galaxies are moving away from Earth.
To astronomers, the wavelength of sunshine a galaxy emits is longer the sooner the galaxy is shifting away from us. The farther away the galaxy is, the extra its gentle has shifted towards the longer wavelengths on the pink facet of the spectrum – so the upper the “redshift.”
As a result of the velocity of sunshine is finite, quick, however not infinitely quick, seeing one thing distant means we’re wanting on the factor the way it regarded up to now. With distant, high-redshift galaxies, we’re seeing the galaxy when the universe was in a youthful state. So “excessive redshift” corresponds to the early occasions within the universe, and “low redshift” corresponds to the late occasions within the universe.
However as astronomers have studied these distances, they’ve discovered that the universe is not only increasing – its fee of enlargement is accelerating. And that enlargement fee is even sooner than the main idea predicts it needs to be, leaving cosmologists like me puzzled and on the lookout for new explanations.
Darkish vitality and a cosmological fixed
Scientists name the supply of this acceleration dark energy. We’re not fairly positive what drives darkish vitality or the way it works, however we predict its habits may very well be defined by a cosmological constant, which is a property of spacetime that contributes to the enlargement of the universe.
Albert Einstein initially got here up with this fixed – he marked it with a lambda in his idea of general relativity. With a cosmological constant, because the universe expands, the vitality density of the cosmological fixed stays the identical.
Think about a field stuffed with particles. If the quantity of the field will increase, the density of particles would lower as they unfold out to take up all of the space within the field. Now think about the identical field, however as the quantity will increase, the density of the particles stays the identical.
It doesn’t appear intuitive, proper? That the vitality density of the cosmological fixed doesn’t lower because the universe expands is, in fact, very bizarre, however this property helps clarify the accelerating universe.
A regular mannequin of cosmology
Proper now, the main idea, or customary mannequin, of cosmology is called “Lambda CDM.” Lambda denotes the cosmological fixed describing darkish vitality, and CDM stands for chilly dark matter. This mannequin describes each the acceleration of the universe in its late phases in addition to the enlargement fee in its early days.
Particularly, the Lambda CDM explains observations of the cosmic microwave background, which is the afterglow of microwave radiation from when the universe was in a “hot, dense state” about 300,000 years after the Huge Bang. Observations utilizing the Planck satellite, which measures the cosmic microwave background, led scientists to create the Lambda CDM mannequin.
Becoming the Lambda CDM mannequin to the cosmic microwave background permits physicists to foretell the worth of the Hubble constant, which isn’t really a relentless however a measurement describing the universe’s present enlargement fee.
However the Lambda CDM mannequin isn’t excellent. The enlargement fee scientists have calculated by measuring distances to galaxies, and the enlargement fee as described in Lambda CDM utilizing observations of the cosmic microwave background, don’t line up. Astrophysicists name that disagreement the Hubble pressure.
The Hubble pressure
Over the previous few years, I’ve been researching ways to elucidate this Hubble pressure. The strain could also be indicating that the Lambda CDM mannequin is incomplete and physicists ought to modify their mannequin, or it might point out that it’s time for researchers to provide you with new concepts about how the universe works. And new concepts are at all times essentially the most thrilling issues for a physicist.
One technique to clarify the Hubble pressure is to switch the Lambda CDM mannequin by altering the enlargement fee at low redshift, at late occasions within the universe. Modifying the mannequin like this will help physicists predict what kind of bodily phenomena is likely to be inflicting the Hubble pressure.
As an illustration, possibly darkish vitality shouldn’t be a cosmological fixed however as a substitute the results of gravity working in new methods. If so, darkish vitality would evolve because the universe expands – and the cosmic microwave background, which reveals what the universe regarded like just a few years after its creation, would have a unique prediction for the Hubble fixed.
However, my team’s latest research has discovered that physicists can’t clarify the Hubble tension simply by altering the enlargement fee within the late universe – this entire class of options falls brief.
Creating new fashions
To check what varieties of options might clarify the Hubble pressure, we developed statistical tools that enabled us to check the viability of the complete class of fashions that change the enlargement fee within the late universe. These statistical instruments are very versatile, and we used them to match or mimic totally different fashions that might doubtlessly match observations of the universe’s enlargement fee and may provide an answer to the Hubble pressure.
The fashions we examined embody evolving darkish vitality fashions, the place darkish vitality acts in a different way at totally different occasions within the universe. We additionally examined interacting darkish energy-dark matter fashions, the place darkish vitality interacts with dark matter, and modified gravity fashions, the place gravity acts in a different way at totally different occasions within the universe.
However none of those might totally clarify the Hubble pressure. These outcomes recommend that physicists ought to examine the early universe to know the supply of the stress.
This text first appeared on The Conversation. You’ll be able to learn the original here.
