Thursday, May 9, 2024

4463 - DARK MATTER - could it be blackholes?

 

-    4463  -   DARK  MATTER  -  could it be blackholes?   -    Tiny black holes left over from the Big Bang may be prime dark matter suspects.  Tiny black holes, created seconds after the birth of the universe, may survive longer than expected, reigniting a suspicion that primordial black holes could account for dark matter, the universe's most mysterious stuff.


---------------------------  4463    -    DARK  MATTER  -  could it be blackholes?

-   Dark matter currently represents one of the most pressing problems in physics. That is because, despite making up an estimated 85% of the matter in the cosmos, dark matter remains effectively invisible to our eyes because it doesn't interact with light.

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-    Because the particles that comprise atoms that compose "everyday" stuff we can see, like stars, planets, and our own bodies, clearly do interact with light, this has prompted the search for dark matter particles outside the Standard Model of particle physics.

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-    Tiny black holes born over 13.8 billion years ago, just after the Big Bang, that are no larger than a proton, could cluster to become suspects for dark matter without the need for new physics.

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-    Not only has a recent change in thinking regarding how black holes "evaporate" prompted a   reassessment of primordial black holes' viability as dark matter suspects, but as the search for a dark matter particle continues to mostly draw a blank, more researchers could begin to look at the primordial black hole dark matter theory more seriously.

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-    Primordial black holes' are a type of black hole that is formed at the beginning of the universe,within the first fraction of a second of the universe.  Galaxies are formed from slight over densities in space present during the early universe.

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-    If the early universe experienced much stronger density fluctuations than the those which created these features, and these fluctuations collapsed at an earlier time than galaxies formed, then those overly dense patches could have spurred primordial black holes.

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-   Depending on the time at which this collapse may have happened as well as the scale of the collapse, these primordial black holes would have very different masses. The primordial black holes would have masses ranging between a few tons and a thousand tons, which is less than the mass of a planet and more in the category of a small asteroid.

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-    Considering how the smallest black holes scientists have discovered to date, known as stellar-mass black holes, have masses equivalent to between 3 and 50 times that of the sun, which itself is 2.2 times 10 to the power of 27 tons, these primordial black holes are incredibly tiny.

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-   Like their larger black hole counterparts formed from either the collapse of massive stars or the merger of relatively smaller black holes, primordial black holes would have a light-trapping outer boundary called an event horizon. The diameter of this horizon is determined by the mass of the black hole, which means the event horizon would be incredibly small in those cases. Smaller than the radius of a proton.

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-   Small, primordial black holes had previously been ruled out as dark matter candidates because all black holes are thought to "leak" a type of thermal radiation first theorized by Stephen Hawking in 1974 and later named "Hawking radiation."

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-   The smaller a black hole, the more rapidly it should leak Hawking radiation and, thus, the faster it should evaporate. That means if primordial black holes ever existed, the smallest examples shouldn't be around today, yet, dark matter clearly is.   Primordial black holes were assumed to have evaporated fully by this time.

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-    If the evaporation process breaks down at a certain point primordial black holes of the masses the scientists considered could achieve a semi-stable state.   In order to decrease its mass through the emission of Hawking radiation, the black hole has to 'rewrite' its information, or something else.

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-    This rewriting process takes time.  It is called 'memory burden' because of this memory that now has to be passed along to something else, and that just kind of slows down the evaporation process overall. So it's a kind of stabilization.  And that means primordial black holes are back as potentially dark matter candidates!

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-    There are other reasons to link these tiny hypothetical black holes to the universe's mysterious matter content.   Perhaps the most obvious connection is dark matter's lack of interaction with light. Dark matter doesn't emit or reflect light, and the event horizon that bounds all black holes represents the point at which the escape velocity necessary to cross it exceeds the speed of light.

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-     That means primordial black holes would "trap" all incident light, resulting in an apparent lack of interactions.   If they are light enough, somewhere around a planetary mass, primordial black holes behave like particles of dark matter.  Dark matter is 'collision-less' in standard models, so dark matter particles do not interact with each other to such a degree that it affects the universe.

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-   If primordial black holes are lighter than planetary masses, then, even on cosmic timescales, they would be so small they'd very rarely collide. These primordial black holes could rather cluster to create the gravitational effects we currently attribute to dark matter, such as providing the gravitational influence that prevents rapidly spinning galaxies from flying apart.

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-    Still, primordial black holes are going to be incredibly difficult to confirm as dark matter, if they really do explain the phenomenon. Again, their light-trapping nature means they are effectively invisible.

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-    At such diminutive sizes, they don't have the same immense gravitational effects as their stellar and supermassive brethren.  Even then, should a cluster of primordial black holes be detected, there is no real way to tell the difference between lots of little black holes and one large black hole.

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-    The results from a pioneering cosmic-mapping project hints that the repulsive force known as “dark energy” has changed over 11 billion years, which would alter ideas about how the Universe has evolved and what its future will be.

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-    The “Dark Energy Spectroscopic Instrument” (DESI) at Kitt Peak Observatory in Arizona is collecting data to reconstruct how the Universe has expanded over billions of years.  The fate of the Universe might not be as dark and empty as cosmologists have long suspected.

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-   DESI started mapping the Universe in 3D in 2020 and was designed to measure the elusive force, known as dark energy, that is pushing galaxies apart.   The surprising early results suggest that dark energy could be weakening over time.

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-   Although the study was based on only the first of the five years planned for data collection, it is already one of the largest maps ever made of the Universe, and it reveals the effects of dark energy across an unprecedented 11 billion years of cosmic history.

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-   If confirmed, the hints that dark energy might be weakening would bring the first substantial change in decades to the generally accepted theoretical model of the Universe. And if dark energy is not constant, that would hold implications for theories of how the Universe has evolved and for what its future might hold.

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-    At the largest scales, the cosmos is ruled by gravity, and Einstein’s general theory of relativity allows for gravity to be repulsive as well as attractive. Whereas ordinary forms of energy, which includes the mass of matter, result in an attractive force, general relativity also predicts that some more-exotic forms of energy could produce “repulsive gravity”.

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-    Dark energy was discovered in 1998, when two teams of astronomers used supernova explosions in distant galaxies to measure how the rate of cosmic expansion has changed. Their results indicated that the rate has accelerated over time, pushed by some unseen repulsive force that would later be dubbed “dark energy”.

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-    The 1998 data that led to the discovery of dark energy had large error bars, and they were consistent with the simplest possible assumption: that dark energy is spread uniformly across space, earning it the name “cosmological constant”, or “Λ”.

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-   A consensus emerged around a theory called “Λ cold dark matter” (ΛCDM), in which cosmic history is largely the result of a struggle between the pull of dark matter and the push of dark energy.

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-    Save for small deviations that remain unexplained, all of the evidence cosmologists have collected so far has strengthened this ΛCDM model. The gold standard was set in 2013 by the Planck space mission of the European Space Agency (ESA), which mapped the relic radiation from the early Universe, called the “cosmic microwave background”. The data from that mission are in “exquisite” agreement with the model.

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-     The current Universe, Planck found, is about 70% dark energy, 25% dark matter and 5% ordinary matter, the stuff of stars, planets and people.

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-    The standard assumption of ΛCDM is that the expansion of the Universe will continue to accelerate, and that most galaxies would ultimately disappear from view. But theorists have developed hundreds of other models of dark energy; many posit that dark energy could be getting slowly diluted, and the Universe’s expansion will start to slow down. Another possibility is that dark energy is getting stronger and will ultimately rip galaxies apart.

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-    For a long time, the hints from observations were too vague to answer even the most basic questions about dark energy: exactly how strong is it, and is it constant or slowly changing? DESI is the first in a new generation of experiments aimed at providing some answers.

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-    Others include ESA’s Euclid mission, which launched into space last year; the massive, 8-meter telescope of the Vera C. Rubin Observatory nearing completion in the Chilean Andes; and NASA’s Nancy Grace Roman Space Telescope, scheduled to launch in 2027. Another telescope, called eROSITA, part of a Russian–German space mission, has mapped the Universe in the X-ray spectrum.

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-    This is really unique in the history of cosmology.   All of these efforts rely on mapping the distribution of matter in the Universe over vast distances, which because of the time that light takes to reach Earth also means over vast stretches of time.

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-    DESI does not take pictures of the sky in the way that an ordinary telescope camera does, but instead collects light from selected locations in its field of view. It does so by pointing optical fibers at objects, typically galaxies or quasars, with its 5,000 robotic arms, and routing that light to sensitive spectrographs. The spectrum of each object reveals its distance, because the farther away the object is, the faster it moves away, and the more its spectrum has ‘redshifted’ towards longer wavelengths.

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-    To reconstruct the history of cosmic expansion from its 3D data, the DESI team uses one of the most well-established techniques in cosmology. It looks at the relic of what used to be “sound waves” in the primordial Universe.

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-    As space expanded and matter cooled over time, the waves became frozen in the distribution of protons and neutrons (known collectively as baryons) across the Universe. That imprint, called “baryon acoustic oscillations”, or BAO, is still detectable today in how galaxies are scattered across space.

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-    DESI doesn’t just see the BAOs in the current Universe. Its 3D map stretches back in time, and by measuring how the average size of the features has grown over time, cosmologists can reconstruct the rate of expansion and from that, the strength of dark energy. The instrument’s results are in principle still compatible with all the options, a dark energy that is constant, weakening or even strengthening.

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-   At the most basic level, the DESI results provide solid confirmation of the original discovery, the Universe is accelerating, and more or less get the same value people have claimed 25 years ago.

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-    The standard model was created as the simplest possible theory for the Universe, but the actual physics of its contents is probably more complicated.

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-    Exotic 'Einstein ring' suggests that mysterious dark matter interacts with itself.  The value for the dark matter mass seems higher than expected.  In the field of one of JWST's largest-area surveys, COSMOS-Web, an Einstein ring was discovered around a compact, distant galaxy. It turns out to be the most distant gravitational lens ever discovered by a few billion light-years.

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-     A fresh analysis of a remarkably massive yet compact galaxy from the early universe suggests that dark matter interacts with itself.   The galaxy, JWST-ER1, which formed just 3.4 billion years after the Big Bang, was first spotted last October in images snapped by NASA's James Webb Space Telescope (JWST).

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-    At over 17 billion light-years from Earth, JWST-ER1g is the farthest-ever example of a perfect "Einstein ring", an unbroken circle of light around the galaxy, a result of light rays from a distant, unseen galaxy being bent due to the space-warping mass of JWST-ER1.

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-   By calculating just how much JWST-ER1g has warped space-time around itself, the discovery team had estimated that the galaxy weighs about 650 billion suns, which makes it a peculiarly dense galaxy for its size. By subtracting the visible stellar mass from the total inferred mass, physicists can measure how much of the galaxy is dark matter, an invisible substance thought to make up over 80% of all matter in our universe.

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-    Despite decades of observations and heaps of circumstantial evidence, this elusive substance is yet to be directly detected. In JWST-ER1g, the discovery team determined that dark matter explains just about half the mass gap, and that "additional mass appears to be needed to explain the lensing results.

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-    JWST-ER1g's unusually high density could be explained by a higher population of stars than currently thought. However, a contraction mechanism by which ordinary matter, the stuff that makes  up gas and stars, collapses and condenses into JWST-ER1g's dark matter halo could be packing more dark matter mass in the same volume, resulting in higher density.

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-   The halo of dark matter, densest at the galaxy's center, is the gravitational glue that prevents spinning galaxies from flying apart. Furthermore, models incorporating a certain type of dark matter, in which its particles interact with themselves, provide an excellent fit to the measurement of JWST-ER1.

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-    We don't yet know what dark matter actually is. Observational clues suggest it is a new kind of particle whose presence can only be inferred from its gravitational interactions with ordinary matter. Dark matter could be just one kind of particle or a complex variety of different types, like in normal matter, that perhaps operates in the presence of additional, hitherto unknown forces exclusive to dark matter.

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-    Last December, 2023, simulations of formations of structures incorporating self-interacting dark matter which concluded that such self-interactions could explain extremely dense dark matter halos in certain galaxies, as well as puzzlingly low densities in others, both of which are unexplained by the prevailing "cold dark matter" theory.

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-   Physicists hope JWST can shed more light on dark matter. The telescope's unprecedented infrared eyes peer further back in time than any other telescope, and its upcoming investigations of galaxies from the very early universe could reveal clues about dark matter particles and their behavior.

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May 9, 2024                DARK  MATTER  -  could it be blackholes?                   4450

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