Recently, the "signal" discovered by searching for dark matter through high-energy physics has aroused people's appetite, which is more likely the result of traditional astrophysics, and it is not the first time to detect the missing mass in the universe, said skeptical astrophysicists.
"10 years ago, no one would rashly say that the signal could not come from a conventional celestial body without first repeatedly verifying it." Stacy McGregor, an astrophysicist at case western reserve university in Cleveland, told Forbes. He said: "But today's attitude seems to be that if you can't immediately recognize what it is, it must be dark matter; The act of falsely reporting that' the wolf is coming' has not been punished. "
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Even so, the theoretical possibility is still very high.
This is because "cold dark matter" in cosmology has been used to explain the gravitational dynamics of many visible substances in the universe for more than half a century. This includes the rotation rate of giant galaxies like our milky way.
"The gravity generated by directly visible substances in galaxies is far from enough to bring galaxies together; If the standard laws of physics are used to calculate gravity, the academic community will introduce dark matter to explain that extra gravity. " Modihai Milgrom, a physicist at the Weizmann Institute in Israel, told Forbes.
Moreover, the "foreign" non-baryon dark matter interacts with conventional matter mainly through gravity in theory, so the detection of dark matter itself is a difficult problem. Even so, most cosmologists admit that traditional dark matter may account for 85% of the missing mass of the universe.
The necessity of introducing dark matter is either related to invisible foreign particles, which is far beyond the scope of known physics, or is the product of new physics. It is believed that the expression of gravity is different in extremely large-scale structures. However, these two hypotheses are not easy to prove.
However, for decades, experimental physicists have searched for this missing part on the ground and in space through laboratory observation and astronomical observation.
An article in Physical Review Letters this month pointed out that the latest observations involved X-rays emitted by perseus cluster and nearby Andromeda galaxy.
Using the XMM- Newton X-ray Space Telescope of the European Space Agency, the particle physics and cosmology laboratory of the Federal Institute of Technology (EPFL) in Lausanne, Switzerland, and Leiden University in the Netherlands, the researchers reported that the observed excessive X-ray photons may be the signal of the decay of inert neutrinos. That is, so far unconfirmed, hypothetical dark matter particles.
"We have been looking for this signal since 2005." Alexey Boyarsky, the first author of the paper and a professor of physics at Leiden University, told Forbes. He also said: "The intensity of the signal is close to the lower limit of the sensitivity of the experimental equipment. If it was easy to find, we would have found it long ago. "
Boyasky pointed out that among all the models consistent with the dark matter interpretation of signals, inert neutrinos may be the simplest and most natural one. He said that such particles only interact with conventional matter through the "stirring" effect with conventional neutrinos in quantum mechanics.
Therefore, Boyasky said, it is difficult to "capture".
Paolo Zuccon, a physicist at Massachusetts Institute of Technology (MIT), put forward a different view that the existence of particles in inertia has not been confirmed. "Its quality, characteristics, especially its decay mode are all guesses," Zuken said in an interview with Forbes. "All in all, this statement is a bit far-fetched."
Or, as McGregor said, "Based on these data, I wouldn't say anything has been detected. This seems to be a typical case of over-interpretation of noisy astronomical data. "
However, Zuken himself has been using a spectrometer erected outside the International Space Station (ISS) to participate in the search for this secret substance.
Zuken and his colleagues analyzed the data collected by Alpha Magnetic Spectrometer (AMS) in two and a half years. This ISS particle detector recorded a large number of cosmic rays from galaxies. They found that the positron exceeded the standard, and the energy of these positrons was around 8 electron volts, which the researchers said was consistent with some dark matter models.
"But we can't distinguish the dark matter hypothesis from astrophysical sources such as pulsars," said Zuken, who participated in the search for dark matter with Alpha Magnetic Spectrometer. "Only when the Alpha Magnetic Spectrometer or other measuring instruments collect more data will the answer be obtained."
However, as reported in Nature News earlier this month, the Planck telescope of the European Space Agency failed to find similar signs of positron surplus in the cosmic microwave background. If dark matter particles collide and annihilate at a similar speed in the primitive universe, they should be found.
McGregor said that as far as the positron signal detected by MIT is concerned, the possible signal from dark matter will correspond to the upper limit of the actual decay energy of dark matter particles.
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"If they see such a steep edge, corresponding to the dark matter particles that seem to exist, I will be very interested," McGregor said. "At that time, there was nothing that astrophysics could not explain."
For a long time, explorers thought that the dense galactic center of the Milky Way was the hiding place of dark matter. At the beginning of this year, using the public data provided by the Fermi Gamma Ray Space Telescope of NASA, researchers found that there were excessive high-energy gamma rays in the center of the Milky Way.
The research on the center of the galaxy has been refined, and the statement that dark matter particles annihilate and release gamma-ray signals has been greatly consolidated. Said Dan Hooper, an astrophysicist at Fermilab in Batavia, Illinois.
Hooper said that independent verification of dark matter signals will require a series of detection results, including: gamma rays from dwarf planet ellipsoid galaxies; Antiproton excess; Gamma rays from sub-halos of galaxies dominated by dark matter; Dark matter particles in underground experiments; Or use the Large Hadron Collider of CERN to get dark matter particles. But he also admitted that the same excess problem can also be explained by phenomena related to pulsars or cosmic ray bursts.
"Only by excluding known and unexpected astrophysical sources, the observed signals are most likely to be emitted by dark matter." McGregor said.
What about Boyasky and his colleagues?
Boyaski pointed out that in 20 15 years, his team gained more time to use XMM telescope. Boyasky said that if this is not enough, it is likely that by the middle of 20 16, the Astro-H X-ray telescope planned by Japan should be able to give his team observation time again to determine whether these X-rays are really emitted by dark matter.
The dark matter theory has always been alive, partly because in the large-scale cosmic structure, invisible dark matter seems to form galaxy clusters and supercluster distributed along the cosmic grid. Therefore, it is difficult to explain such a structure without introducing dark matter or other gravitational theories.
"This optimism about dark matter has existed for a long time," McGregor said. "In the past 20 years, I have heard a confident statement every five years:' In five years, we will know what dark matter is.' Obviously, this day never came. "
Should we stop detecting?
Milgrom said that the detection must continue; Just to show that dark matter doesn't exist.
When does the critical point of bidding farewell to dark matter theory appear?
"For some people, there will never be such a moment." Milgrom said. A few years ago, when he proposed the revised Newton's theory of gravity (MOND), he drew a clear line with the theory of dark matter, which is another theory of gravity and does not need to introduce dark matter to explain anything.
Milgrom said that many years ago, weakly interacting massive particles (WIMP) were expected to be identified as dark matter, but they were not obtained in the large hadron collision experiment.