Editor’s note: Our readers often ask us to do a deep dive into a topic of interest to them. This Big Story was ‘commissioned’ by subscriber Arnab Bhattacharya. If you have a request, be sure to write to us at talktous@splainer.in.

Let’s start at the very beginning…

The big bang: Once upon a time—13.8 billion years ago—our universe was extremely small. Or what physicists call a “singularity”—”a place where matter is compressed down to an infinitely tiny point, and all conceptions of time and space completely break down.” This infinitely tiny and dense point suddenly began to expand—and we mean, suddenly:

Suddenly, an explosive expansion began, ballooning our universe outwards faster than the speed of light. This was a period of cosmic inflation that lasted mere fractions of a second — about 10^-32 of a second.

The core elements—neutrons, electrons and protons—that underlie all creation were formed in a matter of minutes.

Cool point to note: We can’t ever see this early soup because it would take another 400,000 years for those electrons to team up with protons and form atoms of hydrogen and helium. This allowed photons—particles that comprise light—to travel.

That’s the ‘let there be light’ moment—a light also known as the "afterglow" of the Big Bang or the cosmic microwave background (CMB). And this is the cool bit:

According to NASA, CMB fills the universe and in the days before cable TV every household with television could see the afterglow of the Big Bang. By turning the television to an "in-between" channel, you could see the CMB as a static signal on the screen.

Do remember that when you mock your prehistoric elders lol! 

Enter dark matter… 

In the 1920s, great minds like Edward Hubble Albert Einstein proved that the universe is expanding—carrying galaxies further away from us. A decade later, a Swiss-born astronomer Fritz Zwicky pointed out that since the galaxies were moving so fast, they ought to “simply fly apart.” There was some unknown and invisible glue holding them together—which he called “dark matter” (Physicists slap ‘dark’ on to anything they can’t explain—much to the delight of sci-fi writers).

Behold those speeding stars: In the late 1970s, Vera Rubin and Kent Ford confirmed his hypothesis by observing the odd behaviour of outer stars—which ought to orbit more slowly at the edges than those closer to the centre (like planets in our solar system): 

In a spiral galaxy like our own, a couple hundred billion stars orbit around the center. Spirals appear to thin out toward the edges, so the gravitational pull on the outer stars should be weaker out there. Everyone therefore assumed that objects near the edges orbit more slowly. But that is not what I found: Even stars at the periphery are orbiting at high velocities. There has to be a lot of mass to make the stars orbit so rapidly, but we can't see it. We call this invisible mass dark matter.

The galactic superglue: Dark matter is the cement that holds the universe together—acting in tandem with gravity. It is why we have planets, galaxies, solar systems etc. But we don’t know what it is because “it doesn’t reflect, absorb, or emit light”—impossible to see, hard to study.  Another reason: Only 27% of the universe is made of dark matter—and it is spread across vast distances: “[T]here would be so few of these particles in the distances and energies they're involved with, that they're looking for something that's much rarer than a needle in a haystack.”

Enter dark energy…

Nope, not the Darth Vader kind. This is a placeholder term for a universe-shaking discovery made in 1998. They found that the universe was not just expanding–it was speeding up. It was propelled by an unknown anti-gravitational force:

This acceleration implies some unknown force is counteracting gravity to make the universe expand at a greater rate. We call that mysterious force “dark energy”. Despite the name, dark energy isn’t like dark matter, except that they’re both invisible. Dark matter pulls galaxies together, while dark energy pushes them apart.

The Big Freeze cometh:  Dark energy accounts for 68% of the universe’s total mass and energy—and it’s winning the cosmic tug of war with dark matter:

As space expands, matter becomes more spread out, so its influence weakens. By contrast, dark energy is thought to be a property of space itself. So as space expands, it is constantly filled up with more dark energy, so its strength remains constant and it becomes the runaway winner.

Since the universe is expanding at ever faster rates, scientists assumed that it will end in a lonely whimper—aka the ‘big freeze’. Think creation-sized cosmic angst.:

Eventually you have this picture where more distant regions are receding from us faster than light and they become, in principle, untouchable. That loss of contact grows and grows until even the most nearby current galaxies are too far away. It ends in an awful loneliness where we are left isolated because everything is being whisked away too fast.

So distant that even light cannot bridge the gap between galaxies. A universe filled with Matt Damons moping alone—as in ‘The Martian’—in the pitch-black desolation of space.  To summarise: This is what the timeline of creation looks like:

Or maybe not…

Wait, are we slowing down?

Over the past year, scientists at the Dark Energy Spectroscopic Instrument (DESI) near Tucson Arizona have made a startling discovery. Dark energy—thought to be constant—is weakening! Their measurements were taken by 5,000 robotically controlled telescopes capturing 15 million galaxies over 11 billion years of history:

The results suggest that dark energy reached a peak in strength when the universe was about 70% of its current age and it is now about 10% weaker. This would mean the rate of expansion is still accelerating, but that dark energy is gently lifting its foot off the pedal.

In other words, the universe hit peak dark energy billions of years ago. And we’re going to keep slowing down.

What this means: The end of the universe as we know it may look very different–no ‘big freeze’, hello ‘big crunch’. If dark energy dwindles to the point where it becomes negative, the universe will likely end in a “reverse big bang scenario”:

If confirmed, the results.. would have profound implications for theories about the evolution of the universe, opening up the possibility that its current expansion could eventually go into reverse in a “big crunch”.

And the winner of the cosmic tug of war is… dark matter!  

Quote to note: Researcher Mustapha Ishak-Boushaki says of the ‘big crunch’:

It might not seem like the cheeriest fate, but it offers some closure.. “Now, there is the possibility that everything comes to an end,” he said. “Would we consider that a good or bad thing? I don’t know.”

The other possible ending: If dark energy went down to zero—and stayed there without going into the negative—then we would get a new version of the big whimper: “[T]he universe would have a quieter ending, in which galaxies remain in view until stars run out of fuel and flicker out of existence.”

About that ‘so what’ question…

There is no good answer for that in matters of theoretical physics. Presumably, our species will be history long before the universe is either frozen or crunched. But the data—if validated—will upend our understanding of our cosmos—for two reasons.

One: Unlike a big freeze, a big crunch contains the promise of cosmic regeneration, according to DESI researchers: 

We could be back to one of those comforting old solutions where the universe might recollapse – and perhaps start again. I was always disturbed by a universe that keeps expanding forever … to become a dark, frozen expanse. I’m much more at ease with the possibility of new universes emerging in due course.

About those ‘old solutions’: Until now, many physicists pooh-poohed string theory—which posits that all of creation is made up of super tiny strings—not particles. According to an extension of this school of thought—called “cyclic cosmology”—the Big Bang is not a one-off event:

[T]he idea of the universe continually repeating itself is thousands of years old and predates physics, but string theory gave the idea firm mathematical grounding. The cyclic universe goes about exactly as you might imagine, continually bouncing between big bangs and big crunches, potentially for eternity back in time and for eternity into the future.

The big kicker: String theory proponents have long argued that dark energy is not constant—”but must instead take a form called ‘quintessence’—an energy source that will gradually diminish over tens of billions of years.” A conjecture that DESI results seem to confirm. Ergo:

A discovery of quintessence would revolutionize fundamental physics and cosmology, including rewriting the cosmos’s history and future. Instead of tearing apart in a Big Rip, a quintessent universe would gradually decelerate, and in most models, would eventually stop expanding and contract in either a Big Crunch or Big Bounce.

Two: The data raises questions about the very nature of dark energy—which is supposedly the energy of empty space:

The leading theory has long been that empty space itself produces dark energy. Think of a cup of coffee, [Joshua] Frieman said. "If I remove all the particles from the cup of coffee, there is still energy in there due to what we call the quantum vacuum," he said.

But, but, but, if dark energy is weakening—and is not constant then it cannot be "the energy of empty space—because empty space doesn't change." This leads to an even more radical set of possibilities: 

For dark matter to change, it would likely require the existence of some incredibly light, as-yet-unknown particle. Another possibility is that there is something wrong with our calculations—or our understanding of gravity.

In other words, we may have to revisit the most sacred cow of science—Einstein's theory of relativity. We suggest you turn to wiser minds to explain that one.

The most important caveat: When the first DESI results were released a year ago, many scientists were sceptical. It is now clear that changes in dark energy measurements are not a blip. Hence, many of the same experts are backing the data—but absolutely no one knows what it means. Also this: Physics has an extremely high bar for verification called the “five-sigma threshold of statistical certainty”—which we leave CERN to explain. Many more high-powered telescopes—including the European Space Agency’s Euclid mission and the Vera C. Rubin Observatory in Chile—will gather lots more data before we know wtf is happening.

Bottomline: We leave you with this quote from a Lawrence Berkeley National Lab researcher: "We're in the business of letting the Universe tell us how it works, and maybe it is telling us it's more complicated than we thought it was."

Reading list

For the best reporting on the new data, read The Guardian. Phys.org is best on why it matters. Space.com has a bunch of useful explainers on the beginning of the universe (video), Cosmic Microwave Background—and what came before the Big Bang. For more on Dark Matter, read Astronomy Magazine, Centre for Astrophysics—and this Discovery Magazine interview with Vera Rubin. Curious about string theory and where it fit in? Read this older Quanta magazine piece on why string theory is incompatible with dark energy (though not any longer).