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Monday, May 20, 2024
Dr. Brown: Mysteries of dark energy and the expanding universe
It turns out there are more than two trillion galaxies and counting within the visible universe, which some physicists now believe represents but five per cent of the total universe. Richard Harley/Midjourney

Dennis Overbye of the New York Times is one of the best science writers in the world for good reasons: he talks to the principal scientists involved in the subject of interest, other well-qualified scientists not involved in the study and pulls it all together in a way the reader can understand without dumbing the material down too much.

His handling of the question of whether the force that continues to expand the universe is constant or not was well-covered and, along with other tried and true sources, provided the grist for this essay.

To begin, we’ve learned a lot about the universe in the last century. It started out as one galaxy — the Milky Way — but it turns out there are more than two trillion galaxies and counting within the visible universe, which some physicists now believe represents but five per cent of the total universe.

That’s mind boggling, as was earlier evidence that the universe was expanding and had a beginning probably as a tiny nugget of dense energy, which for reasons unknown, almost 14 billion years ago, inflated many times faster than the speed of light to the limit of the visible universe, all within a tiny fraction of the first second.

Then a mist of primordial matter formed and, within a few more seconds, more familiar particles and dark matter were created. That’s a busy paragraph, but so was the universe in those first few seconds of creation.

From that incredibly hot energetic shock-like beginning, the expansion of the universe began to slow and cool enough several hundred thousand years later for stable hydrogen and helium atoms to form — and 100 million years later, for the very first stars and galaxies to form.

In a nutshell that’s what evidence and the standard model of the universe tells us, so far.

Mathematical models are just that: models based on reasonable assumptions and available observations, to account, in this case, for the evolution of the universe and serve as a framework to guide future studies.

But should one or more of those underlying assumptions turn out not to be true based on new observational evidence, it’s time to tweak the model or even toss it out and create a newer model that better fits emerging new observations.

That’s what’s happened to dark energy, the force behind the expansion of the universe. In the standard model, it was assumed to be constant.

The majority of galaxies are speeding away from one another, carried by the continuing expansion of the universe and the velocity at which they are doing so can be measured by employing the doppler effect: those moving away are red-shifted, the more so, the faster their retreating velocities.

To look far out is to look far back in time, and by identifying galaxies of different ages, it should be possible to compare the retreat velocities of galaxies in different time periods.

And If those velocities are similar across different time periods, dark energy can be assumed to be constant. But if the velocities differ between past time periods, suggests that dark energy is not constant, but varies over time.

To answer the question of whether dark energy is constant, a five-year study was recently launched to create a 3-D map of the positions and velocities of 40 million galaxies (sounds like a lot but for perspective, only 0.0000004 per cent of the estimated number of galaxies in the visible universe). Initial data suggests the rate of expansion of the universe may not be constant.

It’s early days in the study, much too early to upset the standard model, but the early observations do raise questions. What if the rate of expansion turns out to be slower or perhaps faster than current models suggest? Or, if the rate of expansion varies, could the continuing expansion of the universe turn into contraction and even a collapse of the universe, as some physicists love to speculate?

This is the grist of theoretical physicists who like to keep all options open regarding the big questions about the eventual fate of the universe. Will we know any better in a few years?

Maybe or never, considering the enormous scope of the universe (much of it out of sight forever because of the enormous distances involved and the fact that the speed of light is too slow for us to ever see those faraway galaxies) — or so we believe.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the InfoHealth series at the Niagara-on-the-Lake Public Library.  

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