For the first time we can see the Universe in a different way. But the new dark matter map is not showing quite what astronomers expected. They have an accurate idea of the distribution of matter , years after the Big Bang, from a European Space Agency orbiting observatory called Planck. It measured the radiation still present from that moment, called the cosmic microwave background, or more poetically, the "afterglow of creation".
Drawing on the ideas of Einstein, astronomers, such as Prof Frenk, developed a model to calculate how matter should disperse over the next But the actual observations from the new map are out by a few per cent - it shows that matter is slightly too evenly spread. As a result, Prof Frenk thinks there may be big changes afoot in our understanding of the cosmos. The current theory rests on very sketchy pillars made of sand.
And what we may be seeing is the collapse of one of those pillars. It seems to pass every test but with some deviations here and there. Maybe the astrophysics of the galaxies just needs some tweaks. In the history of cosmology there are examples where problems went away, but also examples when the thinking shifted. It will be fascinating to see if the current 'tension' in Cosmology will lead to a new paradigm shift," he said. The DES collaboration consists of over scientists from 25 institutions in seven countries.
Follow Pallab on Twitter. In nearly two dozen underground laboratories scattered all over the earth, using vats of liquid or blocks of metal and semiconductors, scientists are looking for evidence of dark matter. Their experiments are getting more complicated, and the search is getting more precise, yet aside from a much-contested signal coming from a lab in Italy, nobody has found direct evidence of the mysterious material that is thought to make up 84 percent of the matter in the universe.
Dark matter is different from regular, baryonic matter — the stuff that makes stars, galaxies, dogs, humans and everything else — in that it does not interact with anything except through gravity and perhaps the weak nuclear force. For many decades, the favored candidates for dark matter particles have been hypothetical shy things called weakly interacting massive particles, or WIMPs.
Many experiments search for them by looking for evidence that a WIMP has come by and knocked regular matter around. In this scenario, a WIMP would tap an atomic nucleus via the weak force.
The startled nucleus would then recoil and emit some form of energy, such as a flash of light or a sound wave. Detecting such barely perceptible phenomena requires sensitive instruments, usually buried deep underground. This is mostly so the instruments are shielded from wayward cosmic rays, which can also cause nuclei recoils. After searching for these faint pings for decades, scientists have little hard evidence to show for it. Now a team of physicists in Poland, Sweden and the U.
A subterranean paleo-detector would work in a manner similar to current direct-detection methods, according to Freese and her colleagues. Instead out outfitting a lab with a large volume of liquid or metal to observe WIMP recoils in real time, they would look for fossil traces of WIMPs banging into atomic nuclei. As nuclei recoil, they would leave damage tracks in some classes of minerals. If the nucleus recoils with enough vigor, and if the atoms that are perturbed are then buried deep in the earth to shield the sample from cosmic rays that can muddy the data , then the recoil track could be preserved.
If so, researchers may be able to dig the rock up, peel away layers of time, and explore the long-ago event using sophisticated nano-imaging techniques like atomic force microscopy. Around five years ago, Freese started tossing around ideas for new detector types with Andrzej Drukier , a physicist now at Stockholm University who began his career studying dark matter detection before pivoting to biophysics.
One of their ideas, devised along with the biologist George Church , involved dark matter detectors based on DNA and enzyme reactions. In Russia, he learned of boreholes drilled during the Cold War, some of which reach 12 kilometers down. No cosmic rays can penetrate that far. Drukier was intrigued. Dark matter could be:.
But all of our efforts to directly detect a candidate particle or field for dark matter have come up empty. We see its astrophysical effects indirectly, and that's indisputable, but on particle-sized scales, we have no idea what's going on. The presence, type, and properties of dark matter clumps can influence the particular variations The fact that we now have detailed spectroscopic data on eight of these systems allows meaningful information to be extracted about the nature of dark matter.
We don't know whether the "dark sector" is simple or rich. Is dark matter, assuming it's made of particles, all made of the same type of particle?
Whether it's all the same component or not, do dark matter particles bind together and form larger, richer structures than merely detached particles? Are there dark atoms, dark molecules, or even larger structures made purely of dark matter out there? We know that dark matter doesn't collide inelastically with itself and lose substantial amounts of angular momentum, but we've only ever probed dark matter structure down to scales of a few thousand light-years.
On scales smaller than that? It's eminently possible that there's an entire dark Universe out there — maybe even including some sort of dark "periodic table" — made of multiple different types of dark particles that interact with one another.
The only restriction is that they do so at a threshold that falls below what we've already placed constraints on. This potential shows an unstable equilibrium point orange ball and a lower, stable equilibrium If the potential then tilts in one direction, that degree of freedom gets removed, and an axion-like particle can all-of-a-sudden get mass from a transition like this.
Did dark matter always exist in the Universe, or was it created at some later time? This is one of the deepest questions we know how to ask, and we do not know the answer. It's possible that dark matter is what's known as a thermal relic, where:. That's dark matter that always existed, as it was created as soon as the hot Big Bang began. But there's another way, emphasized by the above diagram:. This latter scenario corresponds to an axion-like scenario, where these particles both obtain a small but non-zero rest mass and get ripped out of the quantum vacuum in large numbers.
Dark matter may not have always existed, but may have been created later on: before stars formed and before the CMB was emitted, but after the early stages of the hot Big Bang. Is dark matter eternally stable, or will it all someday decay away? This is another situation where all we have are constraints. From the peaks-and-valleys in the cosmic microwave background's fluctuations, we know that dark matter must have existed in a 5-to-1 ratio with normal matter back when the Universe was just a few thousand years old.
From observations of large-scale structure and the centers of galaxies, we know that the dark matter-to-normal matter ratio hasn't changed by any measurable amount over the past But dark matter could decay on timescales longer than the age of the Universe, and we'd have no way of knowing just yet.
Until we know what its properties are, this will remain a mystery. As the ADMX detector is removed from its magnet, the liquid helium used to cool the experiment forms ADMX is the premiere experiment in the world dedicated to the search for axions as a potential dark matter candidate, motivated by a possible solution to the strong CP problem.
Will any of our direct detection experiments ever find it, or is this a fruitless endeavor? Perhaps we're on the cusp of finding an experimental clue as to what dark matter really is. But perhaps not; perhaps all we're going to do is place constraints on the things we know how to measure, like event rates, scattering cross-sections, and potential particle properties and couplings.
We have no way of knowing if the experiments we're performing right now are even capable of revealing dark matter's nature, irrespective of what it is. It's possible we'll get an announcement of a candidate dark matter particle at any point from a variety of experiments, but it's also possible that the ways in which we're presently looking for dark matter will never bear fruit. Nevertheless, we not only know that dark matter exists from the astrophysical evidence, but we've definitively uncovered a large amount of information about what it is, how it behaves, and what it cannot be.
In the quest to understand our Universe, one thing stands out above all others: we must be intellectually scrupulous and honest about what we know, what we don't, and what remains uncertain.
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