- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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."
-
- 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.
-
- 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.
-
- 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!
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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”.
-
- 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”.
-
- 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”.
-
- 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.
-
- 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.
-
- The current Universe, Planck found, is
about 70% dark energy, 25% dark matter and 5% ordinary matter, the stuff of
stars, planets and people.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- The standard model was created as the
simplest possible theory for the Universe, but the actual physics of its
contents is probably more complicated.
-
- 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.
-
- 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).
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
-
May 9, 2024 DARK
MATTER - could it be blackholes? 4450
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