Seen through the eyes of a giant, the galaxies of our universe are gathered in clusters and arrays around inky voids, clinging like foam to the surface of an infinite ocean.

It took eons for this glittering web to come together, slowly coalescing under the guidance of gravity from an evenly distributed fog of white-hot particles fresh from the furnace of the Big Bang billions of years ago.

As slow as this growth may seem to us mere mortals, University of Michigan physicists Nhat-Minh Nguyen, Dragan Huterer and Yuewei Wen want to slow it down even further, and in the process solve one of science's most vexing problems.

Their proposed fine-tuning of the model that currently best describes our Universe could resolve an important contradiction in observations of the expanding waist of space.

No matter how much you complain about getting something for nothing these days, there is more empty space up there today than there was yesterday. Something is causing the nothingness to grow, squeezing through the gaps between galaxies, pushing the large-scale structure of the Universe apart at an ever-increasing rate.

We don't know what's behind this mysterious push, so we call it dark energy.

"If gravity acts like an amplifier that increases perturbations of matter to grow into large-scale structure, then dark energy acts like an attenuator that dampens these perturbations and slows the growth of the structure," says Nguyen, lead author of a study on the growth of large-scale structure.

"By studying how cosmic structure clumps and grows, we can try to understand the nature of gravity and dark energy."

The exact rate of expansion, known as the Hubble constant (H0), is not at all clear. If you measure the way some types of exploding stars are being pulled away, you can get an acceleration of 74 kilometres per second per megparsec. Using the 'light echo' of the stretched radiation still bouncing around after the Big Bang - the cosmic microwave background (CMB) - H0 is closer, at about 67 kilometres per second.

This may not seem like a huge difference, but the discrepancy has persisted through so many investigations that it can no longer be dismissed as a trivial error.

Nguyen, Huterer and Wen have taken a fresh look at the flat ΛCDM-compatible model of cosmology as a potential source of false assumptions. If cosmology were a chess game, this would be the board and the pieces, placed on the pieces of general relativity, moved by the repulsion of dark energy and aligned by the gravitational effects of dark matter.

By rewinding the chess pieces we see today, we can effectively see how the game began, from a momentary blink of an eye of rapid inflation, to a time when the first stars collapsed, to the formation of galaxies and their eventual emergence as gigantic, interconnected threads.

If this process had for some reason deviated from the predictions of the model of harmony and prevented the growth of the large-scale structure of the Universe, the tension between the different measures of the accelerating expansion of the Universe would have disappeared.

Using a combination of measurements including fluctuations in the cosmic web, gravitational lensing events and details in the cosmic microwave background, the researchers reached the statistically convincing conclusion that the cosmic web is growing more slowly than predicted by the flat ΛCDM-consistent cosmology model.

"The difference in these growth rates that we potentially discovered becomes more pronounced as we approach the present day," says Nguyen.

"These different probes individually and collectively point to a growth suppression. Either we are missing some systematic error in each of these probes, or we are missing some new, late-time physics in our standard model."

While there is no clear contender for what could curb the growth of the cosmic web, future measurements of the Universe's large-scale structure could at least hint at whether there is a need to explore this idea further.

It took 13.7 billion years for the Universe to look this good. We can bear to wait a few more years to unravel the secrets of such beautiful-looking cosmological wrinkles.

 

Source: https://www.sciencealert.com/