Could dark matter particles the size of galaxies exist, or a anti-gravitational force field we call “dark energy” that might be getting stronger and denser, leading to a future in which atoms are ripped apart and time ends?
“Our cosmology assumes that most matter comes in a “dark” form that hasn’t yet been detected,” writes Nobel prizewinner Jim Peebles in his article, Why the universe I invented is right – but still not the final answer.
Peebles introduced dark matter and dark energy into our standard model of the cosmos –a picture of a cosmos that some 13.8 billion years ago was in a hot, dense state and has been expanding and cooling ever since, but that is only an approximation to a deeper truth. The current Standard Model relies on Albert Einstein’s cosmological constant, a seemingly arbitrary addition, to explain why the universe’s expansion is apparently speeding up. “Even if you are prepared to overlook these difficulties,” adds Peebles, “there is the unsolved question of what the universe was doing before it was expanding.”
“I have more skin in this game than most, I introduced the mystery elements of dark matter and dark energy into our standard cosmology. So is the model I helped construct right; is our cosmology a true reflection of reality? In what follows, I will strongly argue yes – but only as far as that goes.”
Peebles big idea was that most matter isn’t the “baryonic” matter of the kind you and I and stars and planets are made of. The non-baryonic dark matter I had in mind wouldn’t interact with normal baryonic matter, except through gravity, or with radiation. As it clumped under the influence of gravity, it would slip through the radiation of the cosmic microwave background, leaving it largely undisturbed.
Peebles built his theory two additional hints: First, there was astronomical evidence that most of the mass on the outskirts of galaxies isn’t very luminous. If the visible matter were all that existed, the galaxies would fly apart, based on the speed at which they are rotating.
A Fourth, Heavy Neutrino
The second hint came from growing evidence of a third family, of fundamental particles made up of what became known as the tau and its neutrino. So why not a fourth?
The attraction of a “heavy” fourth neutrino, with a sea of them left from the hot early universe with a mass about three times that of a proton, would provide the matter density required for the universe to be expanding at “escape speed” –the rate of expansion at which the gravity pulling together the universe’s matter is just enough to slow expansion down, but never quite stop it or reverse it back to a “big crunch.”
Cold Dark Matter –Particles the Size of Galaxies?
“Putting the two hints together with the need to account for the quite smooth sea of thermal radiation.” writes Peebles,” resulted in the cold dark matter model. The “cold” refers to the fact that the particles move slowly compared to the speed of light which is tied to the mass of dark matter particles, that is, moving slowly relative to the general expansion of the universe, an important property in the model to ensure the formation of galaxies and clusters of galaxies. The lower the mass of the particle, the ‘warmer’ it is and the faster it will move.
Theoretical physicist Asimina Arvanitaki, at the Perimeter Institute for Theoretical Physics referring to the heated debate about Peeble’s standard model for dark matter that proposes that it is ‘cold, recalls: “At first, we thought it was absurd. How else could you respond to the idea that black holes generate swirling clouds of planet-sized particles that could be the dark matter thought to hold galaxies together? We tend to think about particles as being tiny but, theoretically, there is no reason they can’t be as big as a galaxy.”
On January 9, 2020, NASA physicists using the Hubble Space Telescope announced that although the type of particle that makes up dark matter is still a mystery, a compelling observational test for the cold dark matter passed “with flying colors,” The NASA team used a new “cosmic magnifying glasses” technique that found that dark matter forms much smaller clumps than previously known, confirming one of the fundamental predictions of the widely accepted “cold dark matter” theory.
Physicists at the University of California, Davis, taking the temperature of dark matter, reported that the model of cold (more massive) dark matter holds at very large scales” said Chris Fassnacht, a physics professor at UC Davis, “but doesn’t work so well on the scale of individual galaxies.” That’s led to ruling out other models, including ‘warm’ dark matter with lighter, faster-moving particles and ‘hot’ dark matter with particles moving close to the speed of light.
Vast Halos of Dark Mattter
Astronomical evidence for this dark, gravitating matter is convincing, albeit still not without question. Vast halos of dark matter seem to lurk around galaxies, providing mass that helps hold things together via gravity. On even larger scales, the web-like topography traced by luminous gas and stars also hints at unseen mass.
“Cosmologists usually assume that dark matter has no microstructure,” observes astrophysicist Caleb Scharf. “They think it consists of subatomic particles that interact only via gravity and the weak nuclear force and therefore slump into tenuous, featureless swathes. They have arguments to support this point of view, but of course we don’t really know for sure. Some astronomers, noting subtle mismatches between observations and models, have suggested that dark matter has a richer inner life. At least some component may comprise particles that interact with one another via long-range forces. It may seem dark to us, but have its own version of light that our eyes cannot see.”
Strange, Enduring Mystery of Dark Energy
Speculating about dark energy, physicists have found that for the last 7 billion years or so galactic expansion has been accelerating. This would be possible only if something is pushing the galaxies, adding energy to them. Scientists are calling this something “dark energy,” a force that is real but eludes detection. Powered by dark energy, the cosmos is now doubling in size every 10 billion years — to what end, nobody knows.
“Dark energy is incredibly strange, but actually it makes sense to me that it went unnoticed,” said Noble Prize winning physicist Adam Riess in an interview with The Atlantic. “I have absolutely no clue what dark energy is. Dark energy appears strong enough to push the entire universe – yet its source is unknown, its location is unknown and its physics are highly speculative.”
By 2000, observes Peebles, moving beyond a fourth neutrino, “the data from supernovae in galaxies at different distances pretty convincingly showed that the rate of expansion is not only greater than escape speed, but is also growing over time.” The measurement led to the rebranding of the cosmological constant as dark energy, and later led to the Riess winning the 2011 Nobel prize that was awarded jointly to three members of two competing teams: Saul Perlmutter, Riess and Brian Schmidt.
There are several theories for the identity of dark energy it may be energy generated by ghostly subatomic particles that appear out of nothing before annihilating; it may associated with the Higgs Field, which gives certain kinds of matter mass; or it may be explained by string theory by which extra invisible dimensions of space get compressed into sizes much smaller than atoms. Some theorists believe it’s an example of fine-tuning that demands the existence of a multiverse to explain it.
The Most Dangerous Idea in Physics
“The multiverse may be the most dangerous idea in physics” argues the South African cosmologist George Ellis. The multiverse may be an artifact of a deeper reality that is comprehensible and unique.
A new theory that suggests both dark matter and dark energy can be unified into a fluid which possesses a type of ‘negative gravity’, repelling all other material around them, says Jamie Farnes from the Oxford University e-Research Center. “The outcome seems rather beautiful: dark energy and dark matter can be unified into a single substance, with both effects being simply explainable as positive mass matter surfing on a sea of negative masses.”
About our current ability to make sense of the Universe we live in, Peebles humbly concludes that “I had assembled this cosmology out of the simplest assumptions I thought I could get away with. I can’t have consistently guessed right. Indeed, precise measurements have shown that the initial conditions I assumed – for instance in the detail of how matter warps space-time – were a little out.”