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Saturday, December 2, 2023
Dr. Brown: What is dark energy and is Einstein’s general relativity right?
An AI rendering of Albert Einstein. Richard Harley/Midjourney

One of the things I enjoy and admire about science is that nothing is for certain.

Hypotheses we thought were true may turn out to be wrong, or at least in heavy need of revision in the face of new hypotheses, experiments employing better, more sensitive tools and new evidence.

That’s precisely what many scientists hope will happen following the launch of the European Space Agency’s Euclid Space Telescope in July, a new earth-based telescope in Chile slated to be ready in 2025, and the launch of the Nancy Grace Roman Space Telescope the following year.

In differing ways, these three sophisticated telescopes are designed to tackle the question of what’s behind the expansion of the universe.

That the universe was expanding was suspected first in the 1920s by several theoretical physicists, including Georges Lemaitre, based on their analyses of Albert Einstein’s equations for general theory.

The notion that his equations suggested an expanding universe initially was rejected by Einstein, but prompted him to introduce his famous cosmological constant into his equations to restore stability to the universe.

However, later observations by astronomer Edwin Hubble revealed nearby galaxies were speeding away from one another and the farther away they were, the greater the speed with which they were receding, carried by an expanding universe.

In response, Einstein acknowledged that indeed, the universe was expanding, and withdrew his cosmological constant from his equations for general relativity.

Recent hypotheses and evidence strongly suggest that within the first second following the Big Bang, the universe inflated faster than the speed of light to form a nascent universe to the then visible limit of the universe.

Thereafter the rate of expansion slowed considerably, only to accelerate roughly three billion years following the Big Bang and continuing under the force of an enigmatic expansile force called dark energy.

It turns out the magnitude of this expansile energy is equivalent to Lambda – Einstein’s cosmological constant.

The universe is expected to continue to expand, with no obvious end in sight. At least that’s the current story.

But there are questions. What’s responsible for the expansion?

Does the rate of the expansion described by Lambda work for all times since the beginning of the universe?

Will Lambda’s value continue to govern the future of the universe and theoretically end in a “great tear” in which whole galaxies, stars, planetary bodies and even atoms and subatomic particles are torn apart?

Or, could the period of expansion come to an end sometime in the distant future and even reverse, collapsing the universe into what’s been called the “great gravitational crunch” with everything compressed into a tiny, extremely dense singularity much as it was before the Big Bang?

These are questions theoretical physicists revel in and precisely the ones those very expensive telescopes are designed to answer.

In short, is the cosmological constant, Lambda, an accurate measure of the expansion of the universe?

If not, is Eisenstein’s theory of general relativity in need of revision or even replacement by a new gravitational theory? That’s what’s at stake.

So far, general relativity has stood every test.

What’s not known is whether Einstein’s constant applies to the roughly three billion years following the Big Bang.

The new telescopes aim to see whether the expansion rates of galaxies in that transition period are consistent with Einstein’s cosmological constant.

If so, Einstein’s theory of general relativity and his constant will have withstood yet another challenge.

But if not, it’s back to the theoretical drawing boards to find a new theory to explain the nature of the universe including dark energy.

This is serious stuff, because dark energy comprises 68 per cent of the universe, dark matter another 27 per cent of the universe, leaving ordinary matter made up of subatomic particles and atoms and comprising a little less than 5 per cent.

That’s a lot of mystery in the universe to be solved and the answers won’t be easy or cheap.

Long gone are the small budgets of Rutherford in Cambridge, U.K. or the minuscule budget of an Einstein.

These days, new space and land-based telescopes cost billions of dollars to design, build and maintain.

I think the costs are justified because they answer big questions about the nature of who we are, as well as the universe.

There are other socially intelligent species, but only humans ask existential questions and go about trying to answer them.

Sometimes, with art in all its many forms, other times by devising imaginary, often spiritual creation stories and sometimes using science, and the evidence from all those sources may turn out to be necessary before – as Stephen Hawking speculated – we have a “theory of everything.”  

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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