- 3212 - SUPERNOVA - seen three times? Astronomers saw the same supernova three times thanks to “gravitational lensing“. And, in twenty years they think they’ll see it one more time.
------------------ 3212 - SUPERNOVA - seen three times?
- There are galaxies so far away that the light coming from them can be warped in a way that they actually experience a type of time delay. That is what is happening with extreme forms of gravitational lensing, such as those that give us the beautiful images of “Einstein rings“.
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- The “time dilation” around some of these galaxies can be so extreme that the light from a single event, such as a supernova, can actually show up on Earth at dramatically different times.
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- Finding such a supernova is important not just for its mind bending qualities, it also helps to settle an important debate in the cosmological community. The rate of expansion of the universe has outpaced the rate expected when calculated from the cosmic microwave background radiation.
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- This cosmological mystery is solved by invoking “dark energy”, a shadowy force that is supposedly responsible for increasing the acceleration rate. But, scientists don’t actually know what dark energy is, and to figure it out they need a better model of the physics of the early universe.
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- One way to model this to find an event that is actively being distorted through a gravitational lens. The same event must show up at two separate, distinct times in order to provide input to a calculation about the ratio of the distance between the galaxy doing the lensing and the background galaxy that was the source of the event.
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- That ratio is an important component in calculating some of the variables associated with dark energy. It is only the third such example of a multiply lensed supernova. Quasars have also been caught with their own time delays, but the variable nature of quasars themselves make them less than ideal for the kind of angular distance calculations needed by cosmologists.
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- When the astronomers were surveying data in July 2019, they noted the three point sources of light that were present in data from July 2016 were no longer there. Most likely the data in July 2016 captured a supernova lensed 3 different ways.
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- However, the expected fourth lensing did not show up in the Hubble data. Using their lensing model for the system, the team determined that the fourth image should show up sometime around 2037, plus or minus a few years.
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- With such a long baseline time between appearances of the same event, this supernova would provide valuable data to the debate over time dilation in gravitational lensing events.
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- Unfortunately, that also means that scientists have to wait almost 20 years to get their hands on that data. It also means that they have to keep a watchful eye on that part of the sky in the 2 year window the calculations predict the fourth image of the supernova would appear in.
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- Vera Rubin and Nancy Grace telescopes promise to observe hundreds of these lensed supernovae that can provide even more data to further constrain dark energy. Hopefully they’ll be able to catch the final gasp of the supernova in MRG-M0138 as well.
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- Dark matter may be one of the most mysterious components of our Universe, having eluded direct detection since it was first proposed in the 1930s. Although the astrophysical evidence for its existence is overwhelming from spinning galaxies, galactic motions in clusters, large-scale structure formation, colliding galaxy groups, the cosmic microwave background and more, we don't know what its is.
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- One of the best methods for studying dark matter is through its gravitational effects, particularly in extreme environments: where Einstein's General Relativity makes unique predictions that differ from Newtonian gravity.
- Strong gravitational lensing, where intervening masses between us and a distant source creates distorted, magnified, and multiple images of the target, is one of the best probes of matter in general. With a new set of eight strongly lensed, quadruple-image systems, scientists are learning about dark matter's properties as never before.
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- In Einstein's General Relativity, unlike in Newton's old theory of gravitation, it isn't an invisible attraction between masses that causes what we perceive as gravity, but rather the relationship between matter-and-energy and space-and-time. The presence of matter and energy curves the fabric of space, and that curved space affects everything else in the Universe, including light that passes through that very space.
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- Whenever you have space that's curved by a large enough amount, it will affect the light traveling through that region in a fascinating array of ways. Instead of flat space, where light must always travel in a straight path between two points, the presence of curved space means that multiple paths can be taken to connect two points in space. If the alignment is absolutely perfect, you can even see the background light get stretched into a circular structure: an Einstein Ring.
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- Of course, most of the time the alignment isn't perfect, and there's a good reason perfect alignments are rare.
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- We might think about the Universe as being made of galaxies that are grouped and clustered together into filaments that connect at various nexus points, but that would be a mistake. That is what our Universe appears to look like to our eyes and instruments, but that's only the “normal matter“: the stuff made of protons, neutrons, and electrons.
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- What's unseen by those techniques is the “dark matter“, which is 83% of the mass of the Universe, but only forms the diffuse "skeleton" traced out by the cosmic structure that we can observe.
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- Wherever you have dark matter, it doesn't just make this large, diffuse, fluffy halo on cosmic, super-galactic scales. There are also miniature sub-halos of all different sizes, occurring:
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------------------------- along the filaments,
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------------------------- in the locations where galaxies and clusters form,
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------------------------- between the locations where galaxies exist,
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------------------------- and superimposed atop all the larger structures, both normal and dark.
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- If we were to look at a typical dark matter simulation of a galaxy's halo, and we superimposed the normal, luminous matter atop it, what we would see isn't just one enormous dark matter "fluffball," but a series of smaller-scale dark matter substructure that flows through the galaxy.
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- The gravitational lensing signal that we'll observe is the sum of all the different forms of matter-and-energy that exist along the line-of-sight to a particular object.
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- When you take a look at the details of a system configured like this, it doesn't just depend on the major mass source lensing it, but all of this intricate dark matter substructure arising from these miniature halos as well.
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- By examining exactly how the light from each of the four images is bent relative to one another which is something only newly possible with spectroscopic techniques of ionized oxygen and neon signatures, it's possible to extract information about the types of subhalos that dark matter can form.
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- By observing the variations due to the substructure, which appears at the level of just a few thousandths-of-a-percent, astronomers were able to gain information about the nature of dark matter.
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- We know that dark matter cannot be composed of the known neutrinos present in our Universe: that dark matter would be too hot. While we typically talk about cold dark matter, there is still the possibility that dark matter could be warm at some level, possessing significant kinetic energy for whatever mass it has.
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- From the observations we have, at least 98% of the dark matter must be either cold or warm; hot is ruled out.
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- Tidal streams from the vicinity of the Milky Way provide a probe of substructure and therefore of dark matter's nature, but these streams rely on assumptions the interplay of normal matter with dark matter, which is highly uncertain in a number of regards.
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- The Lyman-alpha forest, where light from distant quasars pass through clouds of gas that partially or wholly absorb the light, enable us to know how small and large structures grow from even very early on in the Universe, but again require assumptions about the gravitational growth of matter and the infall of normal matter into dark matter halos.
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- If dark matter is a thermal relic once produced with the kinetic energy of the other particles in the early Universe, it must be either more massive than 6 keV or 5.3 keV (keV = 1,000 electron volts) from these methods, respectfully. This is some 10,000 times more massive than the current bound on neutrino masses.
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- Imagine that each one of these eight galaxies is a giant magnifying glass. Small dark matter clumps act as small cracks on the magnifying glass, altering the brightness and position of the four quasar images compared to what you would expect to see if the glass were smooth.
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- From just this work, dark matter, if it's a thermal relic, must be more massive than 5.2 keV, meaning it could be either cold or lukewarm, but no hotter.
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- Ever since astronomers first realized that the Universe required the existence of dark matter to explain the cosmos that we see, we've sought to understand its nature. While direct detection efforts have still failed to bear fruit, indirect detection through astronomical observations not only reveal the presence of dark matter, but this novel method of using quadruple lensed quasar systems has given us some very strong, meaningful constraints on just how cold dark matter needs to be.
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- Dark matter that's too hot or energetic cannot form structures below a certain scale, and the observations of these ultra-distant, quadruple-lens systems show us that dark matter must form clumps on very small scales after all, consistent with them being born as arbitrarily cold as we can imagine. Dark matter's not hot, nor can it even be very warm.
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- As more of these systems come in and our instruments go beyond what even Hubble's capabilities are, we might even discover what cosmologists have long suspected: dark matter must not only be cold today, but it must have been born cold. Still to learn “what is it”.
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- July 5, 2021 SUPERNOVA - seen three times? 3212
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