We’re still not sure exactly what dark energy is, but it may have played a key role in the early universe.
Physicists can’t see or measure dark energy (hence the name). The only clue that it exists is how it affects the rest of the universe; dark energy is the force that’s driving the universe to keep expanding faster. Physicists Florian Niedermann of Stockholm University and Martin Sloth of the University of Southern Denmark propose that if dark energy formed bubbles in the dark plasma of the early universe, it could solve one of the biggest mysteries in modern physics.
They describe their idea in a recent paper in the journal Physics Letters B.
What’s New – Physicists have two very reliable ways to measure how fast the universe is expanding, also called the Hubble Constant. They can measure how fast supernovae and galaxies move away from us at different distances, or they can examine the cosmic background radiation left over from the Big Bang. The problem is that these two methods give two different Hubble Constants. But the Hubble Constant is a lot like The Highlander’s Immortals: There can be only one.
The resulting dilemma is called the Hubble tension, and Niedermann and Sloth say they may have found a way to fix it. In computer simulations of the universe, the physicists were able to get the same Hubble Constant from both methods — if a new type of dark energy existed and if it created bubbles in the very early universe.
“It may sound a little scientifically crazy to suggest that something is wrong with our fundamental understanding of the universe, that you can just propose the existence of hitherto unknown forces or particles to solve the Hubble tension,” says Sloth in a statement. “But if we trust the observations and calculations, we must accept that our current model of the universe cannot explain the data, and then we must improve the model.”
Digging Into the Details – Here’s how the idea, which Niedermann and Sloth call Hot New Early Dark Energy, works (hypothetically).
For the first few hundred thousand years after the Big Bang, all the matter and energy that existed — including dark matter and dark energy — were squished together into a hot mess of dense plasma. As the universe slowly cooled, dark energy may have shifted from one phase, or state, to another.
Normal matter changes phases all the time; liquid water freezes into ice or boils into steam, for example. We can’t really picture dark energy freezing or boiling because it’s not really a fluid in the first place; it’s not even normal matter. Instead, Niedermann and Sloth describe dark energy with something physicists call an “equation of state” (don’t panic; there’s no math in this story), which describes a substance’s density, temperature, and other properties. And as the universe cooled off around 380,000 years ago, dark energy’s equation of state suddenly changed.
“The beautiful thing about dark energy is that it is a purely observationally driven theory – we know its there due to its gravitational effects, but we have no idea what it is.”
When a normal fluid cools, it turns into a solid. But when dark energy cooled to a certain point, it started making bubbles — or it could have, according to Niedermann and Sloth, because dark energy is weird and, after all, we can’t say it didn’t.
“The beautiful thing about dark energy is that it is a purely observationally driven theory – we know its there due to its gravitational effects, but we have no idea what it is,” physicist Vivian Sabla of Dartmouth College, who commented on the study, tells Inverse. “As long as a model of dark energy doesn’t contradict the datasets we currently have, it’s still plausible.”
As those bubbles grew, they would have bumped into each other and popped, releasing energy in the process.
And when Niedermann and Sloth factored their bubbling dark energy into simulations of the universe, they got the same Hubble constant from both methods: For every megaparsec between Earth and a distant galaxy, the galaxy moves away from us 72 km/second faster.
Here’s the Background – Niedermann and Sloth aren’t the first to decide that a new form of dark energy that’s doing something unusual in the early universe is the missing piece that could resolve the Hubble tension.
“Early dark energy models have become quite popular as ways to solve the Hubble tension without disagreeing with measurements of the cosmic microwave background (CMB),” explains Sabla.
Here’s why: If you’re trying to measure the Hubble Constant by observing how fast distant galaxies are racing toward the cosmic horizon, you don’t even need to think about the early universe. But if you’re measuring the cosmic background radiation, you need a mathematical description of how the universe evolved.
“If you change the model you use, say from the standard cosmological model, with no early dark energy, to this Hot New Early Dark Energy model, you will extract a different value of the Hubble Constant,” says Sabla. “What the authors have shown in previous papers, and is well-established in other early dark energy models, is that including such a component of dark energy early on in the universe does indeed change the cosmic microwave background-derived value of the Hubble constant such that it agrees with those late-Universe model-independent measurements.”
What’s Next – Hot New Early Dark Energy sounds promising, but it’s not accepted science yet, and it’s competing with a host of other ideas, most of which also involve some form of dark energy doing weird stuff when the universe was young.
“Discovering new fundamental physics carries a heavy burden of proof, so my feeling is that it is probably too early to claim that conclusively,” physicist Marco Raveri of the University of Pennsylvania, who commented on the study, tells Inverse. “We’ll have to wait for more precise data!”
Some of that data may come from telescopes that are now being built, like the European Space Agency’s Euclid mission and the Dark Energy Spectroscopic Instrument (DESI) at Kitt Peak National Observatory in Arizona. Another upcoming survey that may shed light on dark energy is the Sloan Digital Sky Survey’s Baryon Oscillation Spectroscopic Survey (BOSS), which will map large-scale structures in the universe (structures and patterns even larger than galaxy clusters, like a vast cosmic web shaped by gravity).
Models like New Early Dark Energy, and several of the other early dark energy models, usually predict slightly more clustering in these large-scale structures than astronomers actually see in the real universe, so that’s something physicists can look for in future data. If so, it could suggest that the early dark energy explanation either doesn’t work or is still missing a piece.
