The standard model of cosmology – our leading theory of cosmic evolution – suggests that dark energy is constant throughout the universe and over time, making it a fundamental property of space. If it were constant, the mysterious dark energy, which makes up as much as 70 percent of the universe, would push away all stars and galaxies. However, the most comprehensive survey of the universe’s cosmic history suggests that dark energy, also known as the hypothetical “antigravity” force, may evolve over time rather than remain constant, pointing to a less lonely future for the universe’s inhabitants.
It’s been more than two decades since dark energy was discovered. That means scientists have had more than 20 years to unravel the mysteries of this invisible substance that seems to be tearing the universe apart. But they still know next to nothing about it. In fact, dark energy may not even be a substance. It could be a force, or even an intrinsic property of space itself, Space.com reports.
If this preliminary result is confirmed by future observations, cosmologists may have to, at a minimum, examine systematic uncertainties in the prevailing Lambda-CDM (LCDM) model, a mathematical model of the Universe in which lambda represents dark energy. They may also have to start analyzing dozens of other models of our Universe to find the best fit. For now, the data is preliminary – it does not reach the so-called “5 sigma threshold” that determines whether a signal can be considered an official discovery. So the evolving interpretations of dark energy’s evolution may change as new data becomes available in the next few years.
“If this is true, it just turns cosmology on its head,” said Dillon Brout of Boston University, who measures the acceleration of the universe using supernovae. Such a discovery would be “a paradigm shift in our understanding of what our best understanding of the universe is.”
A fragment of the DESI data shows celestial objects from Earth (center) to billions of light-years away. DESI Collaboration/DOE/KPNO/NOIRLab/NSF/AURA/C.Lamman
The Dark Energy Spectroscopic Instrument (DESI), mounted on the Nicholas W. Mayall Four-Meter Telescope at the Kitt Peak National Observatory in Arizona, locates a million galaxies each month. With these observations, cosmologists can measure the expansion rate of the universe over the past 11 billion years. These distant galaxies, which have been likened to the “streetlights of the universe,” help cosmologists study the mystery of dark energy that permeates the universe.
On April 4, 2024, DESI unveiled the largest 3-D map of the Universe ever made. It also includes high-precision measurements of the Universe’s expansion rate over the past 11 billion years. In just its first year of operation, DESI has demonstrated twice the efficiency in measuring the expansion history of the early Universe as its predecessor, the Sloan Digital Sky Survey, which took more than a decade to create a similar 3-D map.
Beyond the countless galaxies clustered together like woven threads, the new 3-D DESI map reveals a subtle pattern in the early universe known as baryon acoustic oscillations (BAOs). These subtle 3-D wrinkles ran through the matter that existed during the first 380,000 years of our universe’s history, solidifying over time into relics of the early cosmos. By mapping the sizes of these frozen BAOs, the researchers were able to estimate the distances to the galaxies and infer the expansion rate of the universe at different points in time.
Because the light from typical galaxies is too faint to be seen because they are so far away and their emission is relatively weak, the DESI collaboration also studied more than 400,000 extremely bright objects called quasars. As they pass through interstellar space, the light from these objects is absorbed by clouds of gas and dust, helping cosmologists map regions of dense matter in a similar way to galaxies.
The preliminary conclusion that dark energy may evolve over time was reached based on an early analysis of the DESI data combined with data from other cosmological sources. The researchers found that the model of evolving dark energy fits the data better than the standard model. It should be noted that no single data set on its own convincingly reveals the evolutionary nature of dark energy, but the signal becomes slightly stronger when all the data sets are combined.
“It’s not a strong enough preference for me to say that the Lambda CDM model is wrong,” Kyle Dawson, co-chair of DESI at the University of Utah, told Space.com. “In fact, we’ve never seen deviations from that model that are meaningful before.”
However, Dawson said, preliminary analysis suggests that dark energy is moving from being a powerful factor accelerating our universe to playing a somewhat diminished role.
“If this is true, we’re in uncharted territory,” Brout said. The DESI collaboration used the second-simplest model of our universe after Lambda CDM, which is unremarkable except for its ability to help cosmologists spot deviations from the standard model. If future observations do show that dark energy evolves over time, dozens of other models will also become viable, and cosmologists will have to test each one individually, Brout said.
Evidence of the universe’s rotation was recently obtained by the James Webb Space Telescope (JWST), which found that two-thirds of galaxies rotate in the same direction. This suggests that there is no randomness and that there is a preferred direction of cosmic rotation.
The universe appears to be rotating, and if so, it could have profound implications for some of science’s biggest questions, including those listed above, according to Polish theoretical physicist Nikodem Poplawski of the University of New Haven, known for his theory that black holes serve as portals to other universes.
“Dark energy is one of the most intriguing mysteries of the universe. Many researchers have tried to explain it by modifying the equations of general relativity or by proposing new fields that could accelerate the expansion of the universe,” Poplawski told Space.com. “It would be surprising if the simple rotation of the universe were the source of dark energy, especially since that predicts its decay.”
In addition, Poplawski noted that other astronomical data seem to show that the angle between the most probable axis of rotation of galaxies and the axis of the main flow of nearby galaxy clusters is 98 degrees, meaning they are almost perpendicular to each other. This is consistent with the hypothesis that the universe is rotating.
To understand why a rotating universe implies the existence of more than one universe, Poplavsky turns to “frames of reference.” These are sets of coordinate systems that are integral to physics and that allow us to measure motion and rest.
Imagine two scientists, Terra and Stella. Each is in a different frame of reference, but Terra is on Earth and Stella is in a spaceship flying past our planet. Terra sees Stella’s frame of reference (the spaceship) moving relative to her own (the Earth), which is at rest. Stella sees her frame of reference as at rest, while Terra’s frame of reference is moving as the Earth moves away.
The expansion of the universe due to dark energy, with galaxies expanding like dots on the surface of an inflating balloon. Robert Lee
Poplavsky noted that if the Universe rotates, then its frame of reference also rotates, and this makes sense only if it rotates relative to at least one other frame of reference.
“If the universe rotates, it must rotate relative to some frame of reference that corresponds to something larger,” he continued. “Therefore, the universe is not unique; it is part of a multiverse.”
According to Poplavsky, the simplest and most natural explanation for the origin of the rotation of the Universe is the cosmology of black holes.
Black hole cosmology suggests that each black hole creates a new daughter universe beyond its event horizon, a one-sided surface that traps light and defines the outer boundary of the black hole.
The theory replaces the central singularity at the heart of the black hole with a “twist in space-time” that generates repulsive gravity, driving the expansion of the new universe.
“Because all black holes are formed from rotating objects, such as rotating stars or the centers of rotating galaxies, they are rotating, too,” Poplawski said. “A universe born in a rotating black hole inherits the black hole’s spin axis as its preferred axis.”
The timeline of the universe and its expansion began in a black hole. NASA/WMAP
In other words, our Universe can rotate in a preferred direction because that is the direction in which the black hole it is sealed inside rotates.
“The black hole becomes an Einstein-Rosen bridge, or wormhole, from the parent universe to the daughter universe,” Poplawski explained. “Observers in the new universe will see the other side of the parent black hole as a primordial white hole.”
Instead of the discovery of a primordial white hole in our Universe leading to our mother black hole and progenitor Universe, the strongest evidence for this black hole cosmology is the preferred direction, or “rotational asymmetry,” in our Universe. This can be seen in the rotational asymmetry of galaxies.
“The motion of individual galaxies in this young universe will depend on its rotation,” Poplawski said. “Galaxies will tend to align their rotation axes with the preferred rotation axis of the universe, which leads to the observed rotational asymmetry.”
Astronomers are already starting to notice this.
Of course, this means that every black hole in our universe is a portal to another young cosmos. These young universes are protected from exploration by the event horizon of their parent black holes, which prevents any signal from coming from the black hole’s interior.
Likewise, traveling through this cosmic portal would be impossible for a budding “multinaut” due to the enormous gravity surrounding the black hole, which would cause tidal forces capable of “spaghettifying” such a brave explorer.
Even if such a multinaut survived the journey, just as nothing can leave a black hole, nothing can enter a white hole, meaning he would have no way to return, no way to report!
But what’s worse is that there’s no guarantee that the laws of physics in the daughter universe will be the same as those in the parent universe, meaning an unpredictable fate and potentially painful death for any brave multinaut who can navigate the black hole.
In any case, before we rush to explore other universes, we have to explore the mysteries of our own universe. And first of all, the mysterious force of dark energy.
Dark energy is a conventional name for any force that causes the Universe to expand at an accelerating rate. Currently, dark energy dominates the Universe, making up 68% of the total amount of cosmic matter-energy. However, this was not always the case.
In the earliest epoch of the universe, the universe was dominated by the energy of the Big Bang, which led to its inflation. As the universe entered the epoch dominated by matter, driven by gravity, this inflation slowed to a near standstill. That should have been the end of the cosmos, but about 9 to 10 billion years after the Big Bang, the universe began to expand again, and this expansion accelerated, leading to the epoch dominated by dark energy.
To understand why this is such a disturbing mystery, imagine giving a child a gentle push on a swing, watching as his swing stops, and then, for no apparent reason, he starts swinging again, and the swing gets faster and faster.
As if dark energy weren’t weird enough, recent results from the Dark Energy Spectroscopic Instrument (DESI) have shown that this mysterious force is weakening. This seems to contradict the standard model of cosmology, or the lambda-cold dark matter (LCDM) model, which assumes that dark energy (represented by the cosmological constant, or lambda) is constant.
The illustration shows matter flowing into a black hole, crossing the Einstein-Rosen bridge, and emerging from a white hole. But does it emerge in a new, nascent universe? Robert Lee
However, Poplawski suggests that a rotating universe could both explain dark energy and explain its weakening.
“Dark energy arises from centrifugal force in a rotating universe on a large scale,” the theoretical physicist explained. “If the universe were flat, the centrifugal force would only act in directions perpendicular to the chosen axis.”
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However, in Poplavsky’s black hole cosmological theory, since the universe created by the black hole is closed, moving in any direction will eventually result in returning from the opposite direction.
This would mean that the centrifugal force arising in the rotating universe becomes a force acting in all directions from the original white hole, the parent universe.
“The magnitude of this force is proportional to the square of the angular velocity of the universe and the distance to the white hole,” Poplavsky said. “This relationship takes the form of a force exerted on the galaxy by dark energy, which is proportional to the cosmological constant and the distance to the white hole. Therefore, the cosmological constant is proportional to the square of the angular velocity of the universe.”
An illustration of our rotating universe as part of a larger multiverse. Robert Lee
But how can this explain the DESI observations, which seem to indicate a weakening of dark energy?
“Since the angular momentum of the universe is conserved, it decreases as the universe expands,” Poplawski said. “Therefore, the cosmological constant, which is the simplest explanation for dark energy, should also decrease with time. This result is consistent with the recent DESI observations.”
To further confirm Poplavsky’s concept, additional data on the flow of galaxy clusters and the asymmetry of the rotation axes of galaxies are needed. This will help to further confirm that our Universe rotates.
Additionally, additional data on the dependence of dark energy on cosmic distances and the passage of time in our 13.7 billion year old cosmos could help confirm whether the weakening of dark energy is related to the decreasing angular velocity of the Universe.
“The next step in developing these ideas is to determine an equation describing the decrease in the cosmological constant generated by the angular velocity of the Universe over time and compare this theoretical prediction with the observed decrease in dark energy,” Poplavsky concluded. “This research may involve searching for a metric that describes an expanding and rotating Universe.”
A new theory claims that dark matter actually shaped the universe. Dark matter is supposed to interact “weakly” with the visible universe, but has this always been the case? Physicists have modeled the inflaton — a theoretical space that scientists use to study the period immediately after the Big Bang — to study these interactions. It’s possible that dark matter interacted with the rest of the universe during this period more evenly than visible matter did.
In a peer-reviewed study, a team of scientists from Northeastern University said dark matter may have played a much bigger role in the birth of our visible universe than we thought. That’s a big claim — physicists have largely considered dark matter to be an unexplained, relatively stable part of the equation, rather than an active participant in shaping what our visible universe became. While scientists generally agree that dark matter is only very, very weakly coupled to the visible universe, these researchers wondered: Is there a place where the two are on equal footing?
It makes intuitive sense that dark matter (which makes up 95% of the modern universe) must have played a role in the Big Bang that gave birth to our cosmos, but it’s deceptively difficult to back up with evidence. Some math and programming courses teach you about “black box functions”: you input a value and see what comes out, but you never really see what process is going on in the middle that leads to the result. The Big Bang may be the ultimate black box function, and scientists are left trying to reverse engineer it using increasingly better hypotheses.
The Big Bang. Flavio Coelho
In their work, PhD student Jinzheng Li and Professor Pran Nath had to simulate both the Big Bang and a range of interactions to reverse engineer the role of dark matter. They tested the idea that the dark and visible sectors are more tightly coupled than we thought – that there might be a stronger coupling between dark matter and normal matter than expected. In physics, coupling is when two particles of any type are bound by one of the four fundamental forces: gravity, electromagnetism, the weak force, or the strong force. When you add dark matter and dark energy into the mix, you also get “weak” interactions, which are much weaker than, well, weak.
Working with weak interactions allows scientists to consider models other than the Standard Model to try to explain the Universe. It’s like substituting 0.0001 to avoid dividing by zero. And if all dark matter were weakly coupled to the visible Universe, that would bolster the idea that the visible Universe has a huge influence on physics. We could continue to use the Standard Model without experiencing any particularly worrisome deviations in mathematical harmony once the influence of dark matter was effectively reduced to zero.
But what if this wasn’t always the case? What if, at perhaps the most crucial moment, dark matter and visible matter were linked more “democratically” – tightly enough that both sides had a more balanced influence on what happens in the universe?
To figure this out, the team explored a theoretical space called the “inflaton,” which scientists use to understand and model the incomprehensibly rapid expansion (or “inflation”) of the ancient universe. If we stop assuming that dark matter was only weakly coupled in the inflaton, suddenly a whole new parameter comes into play. “The hidden sector and the visible sectors could democratically couple to the inflaton,” the team wrote, “in which case the hidden sector and the visible sectors would be in thermal equilibrium at the end of reheating,” or the end of the inflaton.
While this idea may conflict with the Standard Model, the researchers explain that this is precisely why it should interest scientists. “The analysis suggests that the inclusion of hidden sectors that appear in various models of particle physics beyond the Standard Model, and therefore their inclusion, will have implications for the accurate description of physical phenomena,” they conclude.
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