Could Dark Energy Be Changing as the Universe Expands?

Welcome to Bedtime Astronomy. Explore the wonders of the cosmos with our soothing Bedtime Astronomy podcast. Each episode offers a gentle journey through the stars, planets, and beyond, perfect for unwinding after a long day. Let's travel through the mysteries of the universe as you drift off into a peaceful slumber under the night sky.

You know, you're cruising down the highway, maybe doing 65 miles an hour.

Right. And.

Without you even touching the gas, your car just inexplicably accelerates to 100.

Just entirely on its own.

Yeah. And then a few miles later, this like invisible force grabs hold of the chassis and violently throws the car into reverse.

Which would be terrifying.

Completely terrifying. But, I mean, for decades, we've basically believed the universe was operating on that kind of cruise control. You know, driven apart by an unchanging invisible force called dark energy.

Right.

We built our entire understanding of cosmic history on this bedrock idea that dark energy just, well, sits in the background.

Maintaining the exact same strength just forever.

Exactly. But recent observations are really beginning to crack that foundation.

Which is wild. Because what if the universe's biggest mystery isn't constant at all?

And that's exactly what the physics community is facing right now. It's this monumental shift in cosmology.

Yeah.

The data coming in suggests that the fundamental rules governing the expansion of space are actually evolving.

So theoretical physicists are basically scrambling, right, trying to understand if dark energy might have actually flipped its entire nature billions of years ago. Right.

We are moving past just, you know, observing strange phenomena. We're getting into the mechanics of how the actual structural geometry of reality operates.

And it is so easy to think cosmological physics doesn't impact your day-to-day life.

Oh, absolutely.

You might be sitting in traffic right now or folding laundry, wondering what cosmic expansion has to do with you. Right.

It feels very detached.

But this is the fundamental fabric of the space you occupy. We are talking about rewriting the past, the present, and really the ultimate fate of the cosmos that you are a part of.

Yeah. And to understand why this shift is causing such a crisis in physics, we really need to look at the model that's currently breaking down.

The standard model that has ruled cosmology for years.

Exactly.

Okay, so lambda CDM.

Yeah, and it's been incredibly successful, but it's built on some very rigid assumptions.

Let's break that down a bit. So lambda is the Greek letter physicists use as like a mathematical placeholder for the cosmological constant.

Right, which represents dark energy.

And the CDM stands for cold dark matter. Exactly.

And just to clarify that, dark matter is the invisible mass that doesn't interact with light, but it does exert a gravitational pull.

And we call it cold because it moves very slowly, at least compared to the speed of light, right?

Right. And because it's sluggish, it tends to clump together. It forms this invisible gravitational scaffolding that essentially holds galaxies intact.

Okay. So the cold, dark matter is the scaffolding pulling things together. And lambda, the dark energy, is the force pushing empty space apart.

You got it. And in this standard model, lambda is perfectly rigid.

Like a billion years ago, 10 billion years ago, today, it's mathematically treated as identical.

Yeah, it's just a fixed property of space itself.

But a major disruptor recently entered the scene, right? The Dark Energy Spectroscopic Instrument.

Or DESI for short.

DESI.

DESI is just an absolute marvel of modern engineering. I mean, it has spent years creating the largest three-dimensional map of the universe ever attempted.

Which is mind-blowing.

It really is. We are talking about mapping tens of millions of galaxies and quasars to trace the exact history of cosmic expansion over billions of years.

Wait, let me pause you on quasars for a second, because that term gets thrown around in sci-fi a lot.

Oh, totally.

But for anyone who isn't reading astrophysics papers every morning, what actually is a quasar and why is DESE mapping them?

Good question. So a quasar is essentially a cosmic lighthouse. Okay. At the center of massive galaxies in the early universe, there are these supermassive black holes actively gorging on huge amounts of surrounding gas and dust.

And as that material spirals into the black hole, it heats up, right?

Right, to just extreme temperatures. It emits so much energy and light that it literally outshines the entire galaxy around it.

Wow.

Yeah, so because they're so incredibly bright, we can see quasars from billions of light years away.

So by mapping where these quasars are... DESE can trace the structure of the universe in its infancy.

Exactly.

And in March 2025, so after three years of data collection, the DESE collaboration announced findings that just sent shockwaves through the field.

They really did. They combined their massive 3D map with other observations, like the light from distant supernova, the cosmic microwave background, and weak lensing.

Okay, before we get to their actual conclusion, let's clarify weak lensing, because it's crucial to how we measure the invisible. Like, How do you map something you literally can't see?

Well, you look at how it distorts the things you can see.

Okay. Makes sense.

Massive objects like giant clusters of galaxies or just massive clumps of dark matter, they exert tremendous gravity.

Right.

So when light from a more distant galaxy travels past that cluster on its way to our telescopes, the gravity of the cluster actually bends the path of the light.

So the shape of the background galaxy appears slightly stretched or distorted to us? Exactly. Exactly. And by measuring those tiny, subtle distortions across millions of galaxies, which is what weak lensing is, astronomers can map out exactly where the invisible mass is distributed.

So DESI combines their quasar maps with these weak lensing maps, plus the supernova data, and they find strong hints that dark energy's effect has actually evolved over time.

Which completely contradicts the constant part of the cosmological constant.

Right.

I mean.

If dark energy is an inherent property of the vacuum of empty space, how can empty space suddenly change its properties?

And that is the core of the crisis right now. I mean, to be completely rigorous, the DSI data isn't conclusive enough yet to declare an absolute Nobel Prize winning discovery today.

The strength of the statistical evidence kind of varies depending on which data sets you combine, right?

Right, exactly. However, the signal is strong enough that it just cannot be ignored anymore.

The old mathematical framework where dark energy is just this static number bolted onto the equations, it's no longer sufficient.

No, it's not. We need a physical mechanism that explains why the acceleration of the universe would change dynamically.

Which brings us to a really bold approach by an international team led by Professor Özgür Akarsu at Istanbul Technical University. This was published in a 2025 paper in Physical Review D.E.

Yeah, and it's fascinating because when physicists face a crisis where the data doesn't match the model, the historical temptation is to just invent a new imaginary particle.

Right, just bolt it onto the standard model and make the math work.

Exactly. But Akarsu's team went backward. They revisited an existing alternative theory of gravity called teleparallel F of T gravity.

Teleparallel gravity. Okay, to appreciate what that is, we probably have to look at Einstein's general relativity first.

Definitely.

Einstein's masterpiece describes gravity through the curvature of spacetime. Think of spacetime as a tightly stretched rubber sheet, right? Right.

When you place a heavy star or planet on that sheet, it creates a deep valley.

And other objects just naturally roll into that valley.

Exactly. That geometric bending is what we perceive as gravity. In general relativity, matter tells spacetime how to curve, and the curvature of spacetime tells matter how to move.

But teleparallel gravity takes a totally different structural approach.

It does. Instead of curvature, it uses torsion as its foundation.

Torsion. So, twisting.

Yes.

But how does twisting spacetime create gravity?

Okay, imagine taking a thick, wet towel and grabbing both ends. As you begin to wring it out, twisting it in opposite directions, the fabric of the towel binds up.

Tension builds. Right.

The threads pull tighter against one another. Teleparallel gravity posits that mass doesn't create valleys in a rubber sheet. Rather, it creates structural twisting in the fabric of spacetime itself.

And the tension from that twisting creates the forces we measure as gravity? Exactly.

Mathematically, it's entirely equivalent to Einstein's curvature. I mean, it predicts the same planetary orbits.

But the underlying geometry, the structural reality of the universe, is fundamentally different.

Yes, exactly.

And researchers have actually experimented with extensions of this teleparallel theory for years, right? Looking for ways to explain the universe's accelerating expansion?

They have. But when past studies looked at an exponential version of this torsion theory, they ran into a wall of human bias.

They followed a single mathematical path heavily influenced by general relativity.

Yeah, they assumed that the density of dark energy absolutely must always be a positive number.

Because the standard lambda CDM model assumes dark energy density is positive, since that yields a repulsive force.

Right, which matches the acceleration we observe today. So when physicists started exploring alternative theories like torsion, they just brought that assumption with them.

They looked at the equations and basically said, we will mathematically reject any solutions where the dark energy density drops below zero. Exactly.

They put up a roadblock.

But Akarsu's team, which included researchers like Dr. N. Merve Uzun and collaborators from Kyoto University, they completely ignored the roadblock.

They deliberately relaxed that assumption and just let the math dictate the path.

They removed the preconceived human notion of how dark energy should behave.

Right. They ask the equations, if we don't force you to be positive, what do you naturally want to do?

But I have to ask this skeptical question here.

Go for it.

If you just uncap the math and let it take a different path, aren't you just overfitting the data? Like in cosmology, if a model doesn't match the telescope data, theorists sometimes just add new variables, free parameters, until the curve matches.

Right, like adding a fudge factor.

Exactly. How is this not just mathematical gymnastics?

Well, because they didn't add a single free parameter.

Really?

Yeah. In physics, a model's elegance and predictive power are often judged by how few parameters it needs. The more knobs you can turn, the less meaningful the theory is.

Right.

The standard lambda CDM model relies on six fundamental cosmological parameters. This completely overlooked branch of teleparallel gravity, it also uses exactly six parameters. Oh, wow. So they didn't invent a fudge factor at all.

Not at all.

The model naturally dictates which mathematical branch appears based entirely on the physics.

Specifically, the current matter density of the universe interacting with the torsion. Exactly. So what actually happens when we follow this naturally occurring mathematical path? I mean, it results in a completely different timeline for cosmic history, right?

It really does. In this newly explored branch, dark energy's density wasn't just, you know, slightly weaker in the past. It was completely inverted.

It was negative.

Which is crazy to think about.

Right. For the universe's first few billion years, the dark energy density was a negative value. And a negative dark energy density doesn't push empty space apart. It contributes to an attractive force.

So it was acting alongside gravity to pull matter together?

Exactly.

And then, according to this model, a massive cosmic event occurred. The dark energy density crossed zero.

Yes, it happened at around redshift 1.5.

And whenever we talk about looking back in time, redshift comes up. Can you just break down the physical mechanism of redshift for us real quick?

Sure. So as the universe expands, the actual physical space between galaxies is stretching.

Okay.

When a distant galaxy emits light, that light has to travel through this expanding space to reach our telescopes.

And as space stretches, the light waves traveling through it get stretched as well.

Right. Their wavelengths become longer... shifting them toward the red end of the electromagnetic spectrum.

So the further back in time we look, the more the light has been stretched, giving us a higher redshift number.

Exactly. Redshift 1.5 translates to roughly 9.5 billion years ago.

So 9.5 billion years ago, the force governing the cosmos went from negative, crossed zero, and became a positive.

Yeah.

And has it been positive, you know, pushing the universe apart ever since. What actually triggered that flip? I mean, Why would the fabric of reality suddenly change its mind?

It comes down to a cosmic phase transition, which is driven by density.

Okay.

Think about water turning into ice when the temperature hits a critical threshold. In the early universe, matter was packed much closer together. The density of matter was incredibly high.

But as space expanded, that matter spread out, and the overall density of the universe dropped.

Right. According to this teleparallel model, the geometry of spacetime, the torsion, interacts directly with the matter density.

So when the matter density dropped below a specific critical threshold relative to the twisting geometry, it.

Triggered a phase transition in the vacuum of space. The structural tension flipped, and the dark energy effect went from attractive to repulsive.

So we are literally living in the aftermath of a cosmic sign flip. We are. And this lines up perfectly with a concept Akarsu previously developed, right? called the sign-switching lambda CDM model. Yes.

For years, he and his collaborators proved mathematically that a sign-changing dark energy could solve huge observational problems, but they lacked the physical trigger for it.

But now, teleparallel gravity provides the literal mechanism. The sign change emerges naturally from the torsion of spacetime thinning out. Exactly. But, okay, wait. If we are fundamentally tweaking the rules of gravity so that cosmic forces can invert... why doesn't our local physics fall apart?

Right, that's the big question.

Like, why doesn't our own solar system fly apart or crush inward?

Because any new theory of gravity has to pass the solar system test. We know the orbits of the planets with just incredible precision. Right. But the beauty of this torsion model is its built-in shielding mechanism.

Shielding mechanism.

Yeah, the environment dictates the behavior. In highly dense environments like a galaxy or, you know, a solar system packed with a heavy star and planets, the local gravitational field is incredibly strong. That immense local density naturally suppresses the subtle, weird twisting effects of teleparallel gravity. Oh.

So the dense matter acts almost like a heavy knot in the towel, preventing that specific section from twisting strangely.

That is a great way to visualize it, yeah. The twisting effects that cause dark energy to flip, they only really dominate in the vast, empty, ultra-low density voids of deep space on a cosmic scale.

While locally around dense matter, the theory camouflages itself entirely.

Right.

It acts completely indistinguishable from Einstein's general relativity locally.

Which is incredibly elegant on paper. It survives the solar system test. But the true test of this theory is whether it solves the actual observational problems astronomers are facing right now out in the deep universe.

Yes. And that brings us to the most notorious headache in modern cosmology. The Hubble tension.

The Hubble tension. This is a massive disagreement in our measurements of the universe's expansion rate, right?

It is a profound disagreement. When we measure how fast the universe is expanding locally in our present-day cosmic neighborhood using supernovas, we get a specific, very fast speed.

But we also have a way to measure it from the early universe using the cosmic microwave background.

Right, the CMB.

That's the afterglow of the Big Bang, yeah. Correct.

It is the oldest light in the universe. It's essentially a baby picture of the cosmos taken just 380,000 years after the Big Bang.

So we can analyze the patterns in that ancient light, apply the standard constant dark energy model, and predict what the present-day expansion rate should be.

But the numbers don't match.

They don't just slightly disagree, right?

No, they statistically clash in a way that just cannot be written off as an instrument error anymore. The early universe predicts a noticeably slower present-day expansion than what we actually observe happening around us today.

So the standard model simply cannot bridge the gap between the baby picture and the present reality.

Exactly. Astronomers have been arguing for years over whose measurements are wrong.

But this new alternative history based on teleparallel gravity, it changes the conversation entirely. Because if dark energy was negative in the past and positive now, The expansion history of the universe changes completely.

Exactly how the universe expands through its middle ages gets totally rewritten.

So the model accommodates the slower early rate and perfectly bridges it to the higher values of the present-day expansion rate we measure locally.

Right. It solves the tension not by assuming our telescopes are broken, but by accepting that the early universe was temporarily held back by negative dark energy before being released. It is a highly promising physical resolution to the Hubble tension.

So it basically fixes the map by accepting that the roads twist instead of bend and that the speed limit changes over time.

Yeah, that's a perfect analogy.

And the implications go way beyond just fixing a disagreement over past measurements. The extended version of this model accommodates a truly wild possibility hinted at by those recent DESI analyses. Yes.

That cosmic acceleration might actually be slowing down right now.

Which goes against everything we thought we knew.

Right. For a very long time, the depressing assumption was that positive rock energy would just keep accelerating the expansion of the universe faster and faster eternally.

Until every galaxy, star, and atom is eventually ripped apart in a cold void.

Exactly. But this torsion model naturally allows for dynamic changes. It opens the door to the possibility that the acceleration might be hitting the brakes or has even come to a halt in the very recent cosmic past.

So how do we actually prove this? We have this elegant mathematical model. It solves the Hubble tension. But what is the next step to confirm the physics?

We need the full weight of the data. When this study was first published, the researchers were working with preliminary numbers. But DESE has now completed its full five-year survey.

They mapped over 47 million galaxies and quasars.

Right. So the Akarsu team is going to rigorously test this alternative gravity model against that massive data set.

47 million data points tracing the precise structural growth of the cosmos. What specific signatures are they looking for to prove torsion over standard curvature?

They're going to look really heavily at redshift space distortions.

Redshift space distortions. Okay.

As galaxies are pushed apart by cosmic expansion, they are also at the same time falling into the gravitational wells of massive neighboring galaxy clusters.

So that local movement alters their redshift slightly, creating a distortion in the map.

Exactly. And because teleparallel gravity modifies how matter clusters over time compared to standard gravity, it leaves a very specific statistical signature in those distortions.

So if the Torsion model fits the 47 million data points better than the standard model, The textbook literally gets rewritten. It does. I think the philosophical takeaway here is stacked It just shows the sheer necessity of questioning fundamental assumptions.

Absolutely.

For years, brilliant minds left an entire branch of physics sitting in the dark, restricted by this unquestioned bias that dark energy density had to be positive. They looked at the equation through a very specific lens and and basically refuse to let the math guide them into uncomfortable territory.

And you know, science requires immense data collection, but it also occasionally requires stepping back and just interrogating the lens you are using to view that data. Right. Even established, highly successful theories can hide completely unexplored realities if you impose your own constraints on them. Sometimes the breakthrough isn't finding a new particle, it's discovering a new perspective on an old geometry.

So we've moved from a universe with a static, boring, unchanging background energy, to one where gravity is based on the fundamental twisting and torsion of space-time. A universe where a phase transition 9.5 billion years ago flipped dark energy from an attractive force to a repulsive one, finally providing a way to solve the Hubble tension.

It really is a paradigm shift.

But it leaves us with a lingering, almost unsettling reality. This new gravity model suggests that the relentless force pushing our cosmos apart might currently be hitting the brakes. If the dark energy that has been driving expansion for the last 9 billion years is weakening, what does that mean for the ultimate fate of our universe? Will the expansion freeze entirely, leaving a static sky? Or could another phase transition occur, shifting the cosmos into reverse? Next time you look up at the night sky, ask yourself, are we watching a universe expanding endlessly into the dark? Or one that is quietly preparing to pull itself back together?

Could Dark Energy Be Changing as the Universe Expands?

Kapitelklicken springt direkt hin

Shownotes

Transkriptvom Podcast bereitgestellt · Zeitmarken springen hin