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(This message was added in version 6.7.0.) in /home4/scienrds/scienceandnerds/wp-includes/functions.php on line 6114Source:https:\/\/www.quantamagazine.org\/in-a-dark-dimension-physicists-search-for-missing-matter-20240201\/#comments<\/a><\/br> Since then, scientists have wrestled with one striking characteristic of lambda: Its estimated value of 10\u2212122 <\/sup>in Planck units is \u201cthe smallest measured parameter in physics,\u201d said Cumrun Vafa<\/a>, a physicist at Harvard University. In 2022, while considering that almost unfathomable smallness with two members of his research team \u2014 Miguel Montero<\/a>, now at Madrid\u2019s Institute for Theoretical Physics, and Irene Valenzuela<\/a>, currently at CERN \u2014 Vafa had an insight: Such a minuscule lambda is a truly extreme parameter, meaning it could be considered within the framework of Vafa\u2019s previous work in string theory.<\/p>\n Earlier, he and others had formulated a conjecture that explains what happens when an important physical parameter takes on an extreme value. Called the distance conjecture, it refers to \u201cdistance\u201d in an abstract sense: When a parameter moves toward the remote edge of possibility, thereby assuming an extreme value, there will be repercussions for the other parameters.<\/p>\n Thus, in the equations of string theory, key values \u2014 such as particle masses, lambda, or the coupling constants that dictate the strength of interactions \u2014 are not fixed. Altering one will inevitably affect the others.<\/p>\n For example, an extraordinarily small lambda, as has been observed, should be accompanied by much lighter, weakly interacting particles with masses directly linked to lambda\u2019s value. \u201cWhat could they be?\u201d Vafa wondered.<\/p>\n As he and his colleagues pondered that question, they realized that the distance conjecture and string theory combined to provide one more key insight: For these lightweight particles to appear when lambda is almost zero, one of string theory\u2019s extra dimensions must be significantly larger than the others \u2014 perhaps large enough for us to detect its presence and even measure it. They had arrived at the dark dimension.<\/p>\n To understand the genesis of the inferred light particles, we need to rewind cosmological history to the first microsecond after the Big Bang. At this time, the cosmos was dominated by radiation \u2014 photons and other particles moving close to the speed of light. These particles are already described by the Standard Model of particle physics, but in the dark dimension scenario, a family of particles that are not a part of the Standard Model can emerge when the familiar ones smash together.<\/p>\n \u201cEvery now and then, these radiation particles collided with each other, creating what we call \u2018dark gravitons,\u2019\u201d said Georges Obied<\/a>, a physicist at the University of Oxford who helped craft the theory of dark gravitons<\/a>.<\/p>\n Normally, physicists define gravitons as massless particles that travel at the speed of light and convey the gravitational force, similar to the massless photons that convey the electromagnetic force. But in this scenario, as Obied explained, these early collisions created a different type of graviton \u2014 something with mass. More than that, they produced a range of different gravitons.<\/p>\n \u201cThere is one massless graviton, which is the usual graviton we know,\u201d Obied said. \u201cAnd then there are infinitely many copies of dark gravitons, all of which are massive.\u201d The masses of the postulated dark gravitons are, roughly speaking, an integer times a constant, M<\/em>, whose value is tied to the cosmological constant. And there\u2019s a whole \u201ctower\u201d of them with a broad range of masses and energy levels.<\/p>\n To get a sense of how this all might work, imagine our four-dimensional world as the surface of a sphere. We cannot leave that surface, ever \u2014 for better or worse \u2014 and that\u2019s also true for every particle in the Standard Model.<\/p>\n Gravitons, however, can go everywhere, for the same reason that gravity exists everywhere. And that\u2019s where the dark dimension comes in.<\/p>\n To picture that dimension, Vafa said, think of every point on the imagined surface of our four-dimensional world and attach a small loop to it. That loop is (at least schematically) the extra dimension. If two Standard Model particles collide and create a graviton, the graviton \u201ccan leak into that extra-dimensional circle and travel around it like a wave,\u201d Vafa said. (Quantum mechanics tells us that every particle, including gravitons and photons, can behave like both a particle and a wave \u2014 a 100-year-old concept known as wave-particle duality.)<\/p>\n As gravitons leak into the dark dimension, the waves they produce can have different frequencies, each corresponding to different energy levels. And those massive gravitons, traveling around the extra-dimensional loop, produce a significant gravitational influence at the point where the loop attaches to the sphere.<\/p>\n \u201cMaybe this is the dark matter?\u201d Vafa mused. The gravitons they had concocted were, after all, weakly interacting yet capable of mustering some gravitational heft. One merit of the idea, he noted, is that gravitons have been a part of physics for 90 years, having been first proposed as carriers of the gravitational force. (Gravitons, it should be noted, are hypothetical particles, and have not been directly detected.) To explain dark matter, \u201cwe don\u2019t have to introduce a new particle,\u201d he said.<\/p>\n Gravitons that can leak into the extra-dimensional domain are \u201cnatural candidates for dark matter,\u201d said Georgi Dvali<\/a>, director of the Max Planck Institute for Physics, who is not working directly on the dark dimension idea.<\/p>\n A large dimension such as the posited dark dimension would have room for long wavelengths, which imply low-frequency, low-energy, low-mass particles. But if a dark graviton leaked into one of string theory\u2019s tiny dimensions, its wavelength would be exceedingly short and its mass and energy very high. Supermassive particles like this would be unstable and very short-lived.\u00a0 They \u201cwould be long gone,\u201d Dvali said, \u201cwithout having the possibility of serving as dark matter in the present universe.\u201d<\/p>\n Gravity and its carrier, gravitons, permeate all the dimensions of string theory. But the dark dimension is so much bigger \u2014 by many orders of magnitude \u2014 than the other extra dimensions that the strength of gravity would get diluted, making it appear weak in our four-dimensional world, if it were seeping appreciably into the roomier dark dimension. \u201cThis explains the extraordinary difference [in strength] between gravity and the other forces,\u201d said Dvali, noting that this same effect would be seen in other extra-dimensional scenarios<\/a>.<\/p>\n Given that the dark dimension scenario can predict things like dark matter, it can be put to an empirical test. \u201cIf I give you some correlation you can never test, you can never prove me wrong,\u201d said Valenzuela, a co-author of the original dark dimension paper<\/a>. \u201cIt\u2019s much more interesting to predict something that you can actually prove or disprove.\u201d<\/p>\n Astronomers have known dark matter existed \u2014 at least in some form \u2014\u00a0since 1978, when the astronomer Vera Rubin established that galaxies were rotating so fast that stars on their outermost fringes would be cast off into the distance were it not for vast reservoirs of some unseen substance holding them back. Identifying that substance, however, has proved very difficult. Despite nearly 40 years of experimental efforts to detect dark matter, no such particle has been found.<\/p>\n If dark matter turns out to be dark gravitons, which are exceedingly weakly interacting, Vafa said, that won\u2019t change. \u201cThey will never be found directly.\u201d<\/p>\n But there may be opportunities to indirectly spot the signatures of those gravitons.<\/p>\n One strategy Vafa and his collaborators are pursuing draws on large-scale cosmological surveys that chart the distribution of galaxies and matter. In those distributions, there might be \u201csmall differences in clustering behavior,\u201d Obied said, that would signal the presence of dark gravitons.<\/p>\n When heavier dark gravitons decay, they produce a pair of lighter dark gravitons with a combined mass that is slightly less than that of their parent particle. The missing mass is converted to kinetic energy (in keeping with Einstein\u2019s formula, E<\/em> = mc<\/em>2<\/sup>), which gives the newly created gravitons a bit of a boost \u2014 a \u201ckick velocity\u201d that\u2019s estimated to be about one-ten-thousandth of the speed of light.<\/p>\n These kick velocities, in turn, could affect how galaxies form. According to the standard cosmological model, galaxies start with a clump of matter whose gravitational pull attracts more matter. But gravitons with a sufficient kick velocity can escape this gravitational grip. If they do, the resulting galaxy will be slightly less massive than the standard cosmological model predicts. Astronomers can look for this difference.<\/p>\n Recent observations of cosmic structure from the Kilo-Degree Survey are so far consistent with the dark dimension: An analysis of data from that survey placed an upper bound<\/a> on the kick velocity that was very close to the value predicted by Obied and his co-authors. A more stringent test will come from the Euclid space telescope, which launched last July.<\/p>\n Meanwhile, physicists are also planning to test the dark dimension idea in the laboratory. If gravity is leaking into a dark dimension that measures 1 micron across, one could, in principle, look for any deviations from the expected gravitational force between two objects separated by that same distance. It\u2019s not an easy experiment to carry out, said Armin Shayeghi<\/a>, a physicist at the Austrian Academy of Sciences who is conducting the test. But \u201cthere\u2019s a simple reason for why we have to do this experiment,\u201d he added: We won\u2019t know how gravity behaves at such close distances until we look.<\/p>\n The closest measurement to date<\/a> \u2014 carried out in 2020 at the University of Washington \u2014 involved a 52-micron separation between two test bodies. The Austrian group is hoping to eventually attain the 1-micron range predicted for the dark dimension.<\/p>\n While physicists find the dark dimension proposal intriguing, some are skeptical that it will work out. \u201cSearching for extra dimensions through more precise experiments is a very interesting thing to do,\u201d said Juan Maldacena<\/a>, a physicist at the Institute for Advanced Study, \u201cthough I think that the probability of finding them is low.\u201d<\/p>\n
\nIn a \u2018Dark Dimension,\u2019 Physicists Search for the Universe\u2019s Missing Matter<\/br>
\n2024-02-05 21:58:24<\/br><\/p>\nThe Dark Tower<\/strong><\/h2>\n
Riddles of the Dark<\/strong><\/h2>\n