Tag: Dark matter

Astrophysicists Spotted A ‘Galaxy Without Dark Matter’

An unusual galaxy far, far away is stumping astronomers not because of what’s there, but because of what’s missing.

About 65 million light-years away, the galaxy called NGC1052-DF2 is dim and diffuse, coming in at about one two-hundredths the mass of our Milky Way.

Normally, not all of a galaxy’s mass is visible. In addition to a mix of ordinary matter—like stars and planets and manatees—galaxies are expected to contain dark matter, an invisible substance that makes up most of the mass in the universe.

Although we can’t directly observe it, we know dark matter is there because we can see how its gravity affects ordinary matter.

Based on the ratio in other galaxies, an isolated galaxy like NGC1052-DF2 should have about a hundred times more dark matter than ordinary matter. But this one appears to have … almost none, scientists report today in Nature.




How did scientists figure that out?

Using a cluster of lenses called the Dragonfly Telephoto Array, a team led by Yale University’s Pieter van Dokkum took a really close look at NGC1052-DF2.

By tracking the motion of 10 embedded star clusters, the team could determine how much mass is tucked into the galaxy. And surprisingly, it’s about the same amount of mass they’d expect to see from the galaxy’s stars alone.

We really thought dark matter was not just an optional component of galaxies,” van Dokkum says, noting that the team has found several other similarly perplexing galaxies to scrutinize.

Why is this observation important?

One strange observation doesn’t necessarily break a theory. But finding a galaxy that’s more or less devoid of dark matter certainly suggests a few tantalizing things. First, it really challenges ideas about how galaxies form.

In modern galaxy formation theory, our understanding is that galaxies form in a dark matter halo,” says Stanford University’s Risa Wechsler.

There’s a pretty tight relationship between the amount of stars that formed and the dark matter there, at least when the galaxy formed.

In other words, no dark matter, no galaxy.

In theories proposing alternatives to dark matter, such as modifications to our understanding of gravity, whatever is mimicking the dark matter signature is not something that can be turned on or off—it should always be there.

So, van Dokkum says, “by not detecting the dark matter, we actually prove it’s real.”

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Astronomers Found Evidence For A ‘Dark’ Gravitational Force That Might Fix Einstein’s Most Famous Theory

Albert Einstein’s general theory of relativity predicts so much about the universe at large, including the existence of gravitational lenses or “Einstein rings.”

And yet his famous equations struggle to fully explain such objects.

While general relativity says a strong source of gravity — like the sun— will warp the fabric of space, bend light from a distant object, and magnify it to an observer, very big objects like galaxies and galaxy clusters make gravitational lenses that are theoretically too strong.




General relativity also can’t fully explain the spinning motions of galaxies and their stars.

That’s why most physicists think as much as 80% of the mass in the universe is dark matter: invisible mass that hangs out at the edges of galaxies.

Dark matter might be made of hard-to-detect particles, or perhaps an unfathomable number of tiny black holes. But we have yet to find smoking-gun evidence of either.

However, a contentious theory by Erik Verlinde at the University of Amsterdam suggests dark matter may not be matter at all.

What’s more, astronomers say his idea “is remarkable” in its ability to explain the behavior of more than 33,000 galaxies that they studied.

This does not mean we can completely exclude dark matter, because there are still many observations that Verlinde’s theory cannot yet explain,” study leader and physicist Margot Brouwer said in a YouTube video about the research.

However it is a very exciting and promising first step.”

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Universe’s First Stars Detected? Here Are The Facts!

Stars are our constant companions in the night sky, but seas of twinkling lights weren’t always a feature of the cosmos.

Now, scientists peering back into deep time suggest that the earliest stars didn’t turn on until about 180 million years after the big bang, when the universe as we know it exploded into existence.

For decades, teams of scientists have been chasing—in fact, racing—to detect the signatures of these first stars.

The new detection, from a project called EDGES, is in the form of a radio signal triggered when light from those stars began interacting with the hydrogen gas that filled primordial empty space.

If the signal stands up to scrutiny, the detection simultaneously opens up a new line of cosmological inquiry and offers a few conundrums to tackle.

The era of cosmic dawn has been entirely uncharted territory until now,” says physicist Cynthia Chiang of the University of KwaZulu-Natal in South Africa.

It’s extremely exciting to see a new glimpse of this slice of the universe’s history, and the EDGES detection is the initial step toward understanding the nature of the first stars in more detail.




Cosmic Dawn

Shortly after the universe was born, it was plunged into darkness. The first stars turned on when hot gas coalesced around clumps of dark matter, then contracted and became dense enough to ignite the nuclear hearts of infant suns.

As those early stars began breathing ultraviolet light into the cosmos, their photons mingled with primordial hydrogen gas, causing it to absorb background radiation and become translucent.

When that happened, those hydrogen atoms produced radio waves that traveled through space at a predictable frequency, which astronomers can still observe today with radio telescopes.

The same process is going on in modern stars as they continue to send light into the cosmos.

But the radio waves produced by those first stellar gasps have been traveling through space for so long that they’ve been stretched, or redshifted.

That’s how astronomers identified the fingerprints of the earliest stars in radio waves detected by a small antenna in western Australia.

From Light to Dark

If the signal is real, it presents a challenge for some scientists who’ve been thinking about how the early universe worked.

For starters, the time frame during which these earliest stars emerged lines up well with some theories, but it’s not exactly bang on with others.

In previous work, Furlanetto and his colleagues started with actual observations of the earliest known galaxies, and then rewound the cosmic clock using computer models, searching for the age at which a signal from the first stars might appear.

The universe’s first galaxies are thought to be small, fragile, and not that great at birthing stars, so Furlanetto wouldn’t expect the signal to peak until about 325 million years after the big bang.

But if the first stars had already furnished enough light to make their presence known 180 million years after the big bang, those early galaxies must be doing something different.

As well, the primordial hydrogen gas is absorbing photons at rates that are at least two times higher than predicted.

That’s problematic for some ideas about the temperature of the early universe. It means that either the primordial gas was colder than expected, or background radiation was hotter.

Dark matter makes up the bulk of the universe’s mass, but it doesn’t behave like normal matter and has proven tricky to understand.

It regularly evades direct detection, and scientists are struggling to pin down, what, exactly it is and how it has influenced the structure of the universe through time.

But, she notes, it’s way too early to accept that conclusion.

An alternate possibility is that there are simply more photons for the hydrogen gas to absorb, though it’s not obvious where all those photons would come from in the early universe.

So she and others are waiting for independent confirmation of the EDGES result before diving too deep into the possible dark matter scenarios.

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Black Holes Destroy Dark Matter And Emit Gamma Rays

Black holes can cause dark matter to annihilate in their vicinity by concentrating the dark matter and enhancing the collision rate between dark matter particles.

The best observational candidates are supermassive black holes, such as the 4 million solar mass black hole found at the center of our Milky Way galaxy.

Some galaxies have much larger supermassive black holes, reaching as high as several billion or even tens of billions of solar masses. Most massive galaxies appear to have supermassive black holes in their centers.

We infer the existence of supermassive black holes through their effect on nearby stellar or molecular cloud orbits.




And we more directly detect supermassive black holes (SMBHs) by the radiation emitted from ordinary matter that is near the black hole (BH), but has not yet fallen into the BH’s event horizon.

Such matter will often form a hot accretion disk around the SMBH.

The disk or other infalling matter can be heated to millions of degrees by the strong gravitational potential of the BH as the kinetic energy of infall is converted to thermal energy by frictional processes.

Ordinary matter (OM) heated to such high temperatures will give off X-rays.

Now if OM is being pulled into a SMBH, so is dark matter, which pervades every galaxy. Dark matter (DM) responds to the same gravitational potential from the SMBH.

The difference is that OM is collisional since it easily interacts with other OM via the electromagnetic force, whereas DM is generally collisionless, since it does not interact via electromagnetism.

Nevertheless DM – DM collisions can occur, rarely, via a ‘direct hit’ (as if two bullets hit each other in mid-air) and this leads to annihilation.

Two DM particles meet directly and their entire energy content, from their rest mass as well as their kinetic energy of motion, is converted into other particles.

The cross-section strength is not known, but it must be small due to observational limits, yet is expected to be non-zero. The most likely candidates for decay products are expected to be photons, neutrinos, and electrons.

The leading candidate for DM is some sort of weakly interacting massive particle with a mass of perhaps 5 to 300 GeV; this is the range where DM searches from satellites and on Earth are focused.

So if two DM particles mutually annihilate, there is of order 10 GeV to 600 GeV of available rest mass energy to produce highly energetic gamma rays.

The likelihood of a direct hit is proportional to the square of the density of the DM.

A SMBH’s gravitational potential acts as a concentrator for DM, allowing the density to be high enough that there could be a significant number of annihilation events, resulting in a detectable flux of escaping photons reaching Earth.

Relativistic effects work to further increase the annihilation rate. And it is possible that the annihilation signal could scale as M³ (mass of the SMBH cubed), and thus the most massive SMBHs would be very strong gamma ray emitters.

These would be highly energetic gamma rays with well over 1 GeV of energy.

The search for gamma rays from annihilating DM around SMBHs is already underway. There is in fact a possible detection by the Fermi telescope at 130 GeV in our Milky Way galaxy, from the direction of the Sagittarius A* SMBH.

Future more sensitive gamma ray surveys may lead to many detections, helping us to better understand both dark matter and black holes.

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The Universe May Be Expanding Faster Than We Thought. Does It Mean Something?

At the beginning of time, all the matter in the universe was compressed into an infinitesimally small point. That tiny speck of everything then exploded and formed the universe.

In some sense, it’s still exploding, expanding at an accelerating rate.

In the past, scientists have looked to the radiation left behind from the Big Bang — its smoking gun — to calculate what the rate of the expanding universe ought to be today.

But new evidence, soon to be published in The Astrophysical Journal, suggests these estimates may be wrong, or at least incomplete.

New observations from the Hubble Space Telescope have indicated that the universe may be expanding 5 to 9 percent faster than predicted by the Big Bang.




But how?

Using the Hubble, scientists from across the US were able to painstakingly measure the distance to stars and supernovae in many galaxies.

They then used this data to refine what’s known as the “Hubble constant,” the rate by which the universe expands, as measured by direct observations.

But when this new “Hubble constant” was compared with the estimates from the Big Bang inferences, the numbers just didn’t match.

You start at two ends, and you expect to meet in the middle if all of your drawings are right and your measurements are right,” Adam Riess, the Nobel laureate at the Space Telescope Science Institute and Johns Hopkins University, who led the project, explained Thursday in a statement.

“But now the ends are not quite meeting in the middle and we want to know why.”

Add this to the long list of questions physicists still have about the universe

The prediction based on the Big Bang “should match our measurement,” Lucas Macri, a Texas A&M physicist and one of the study’s co-authors, tells me.

“If they don’t … there must be a physical reason why these two things are not agreeing.”

So what accounts for the discrepancy?

Either there’s something about the Big Bang that previous estimates have not accounted for or there are factors that come into play after the Big Bang that scientist don’t yet understand.

Macri highlights four possible explanations.

The first is related to the Big Bang.

)We’re seeing evidence of a previously unknown subatomic particle that was abundant right after the Big Bang (a.k.a. ‘dark radiation’),” he says.

If you change the assumptions about what was in the primordial soup, things will have shifted a bit.

The other possibilities are related to “dark energy” and “dark matter,” the substances that make up most of the universe yet can’t be directly observed.

2) Dark energy — the mysterious force that opposes gravity and is causing the universe to accelerate — “is growing in strength and ‘pushing’ galaxies apart faster than it did before,” he says.

3) Dark matter — matter that we can’t see but that is theorized to exist and make up most of the matter in the universe — “is even weirder than we thought.

Or it could not so simply be:

4)Our theory of gravity is incomplete.”

He also mentions that their results aren’t set in stone. “There’s one chance in 1,000 that we got this measurement by accident,” he says.

Physics requires a one in 4 million chance for results to be considered truth. More observations will need to be made.

Macri says he and other researchers will know more soon, especially if they get to use the James Webb Space Telescope, which will replace Hubble in the year 2018.

The James Webb will be able to look much deeper into space than Hubble and can refine the Hubble constant estimate further.

A modest amount of time with James Webb will allow us to make a very significant improvement on our measurement,” Macri says.

Overall, he says, it’s important to know the exact rate of universal expansion because it will yield a more accurate age of the universe.

To get the age of the universe you need to have the Hubble constant,” he says. Right now the uncertainty of their estimate is 2.4 percent, which is the best yet. But not good enough.

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Mysterious Dark Matter May Not Always Have Been Dark

The nature of dark matter is currently one of the greatest mysteries in science. The invisible substance — which is detectable via its gravitational influence on “normal” matter — is thought to make up five-sixths of all matter in the universe.

Astronomers began suspecting the existence of dark matter when they noticed the cosmos seemed to possess more mass than stars could account for.

For example, stars circle the center of the Milky Way so fast that they should overcome the gravitational pull of the galaxy’s core and zoom into the intergalactic void.

Most scientists think dark matter provides the gravity that helps hold these stars back.




Scientists have mostly ruled out all known ordinary materials as candidates for dark matter. The consensus so far is that this missing mass is made up of new species of particles that interact only very weakly with ordinary matter.

One potential clue about the nature of dark matte rhas to do with the fact that it’s five times more abundant than normal matter, researchers said.

This may seem a lot, and it is, but if dark and ordinary matter were generated in a completely independent way, then this number is puzzling,” said study co-author Pavlos Vranas, a particle physicist at Lawrence Livermore National Laboratory in Livermore, California.

Instead of five, it could have been a million or a billion. Why five?

The researchers suggest a possible solution to this puzzle: Dark matter particles once interacted often with normal matter, even though they barely do so now.

The protons and neutrons making up atomic nuclei are themselves each made up of a trio of particles known as quarks.

The researchers suggest dark matter is also made of a composite “stealth” particle, which is composed of a quartet of component particles and is difficult to detect.

The scientists’ supercomputer simulations suggest these composite particles may have masses ranging up to more than 200 billion electron-volts, which is about 213 times a proton’s mass.

Quarks each possess fractional electrical charges of positive or negative one-third or two-thirds. In protons, these add up to a positive charge, while in neutrons, the result is a neutral charge.

Quarks are confined within protons and neutrons by the so-called “strong interaction.

The researchers suggest that the component particles making up stealth dark matter particles each have a fractional charge of positive or negative one-half, held together by a “dark form” of the strong interaction.

Stealth dark matter particles themselves would only have a neutral charge, leading them to interact very weakly at best with ordinary matter, light, electric fields and magnetic fields.

The researchers suggest that at the extremely high temperatures seen in the newborn universe, the electrically charged components of stealth dark matter particles could have interacted with ordinary matter.

However, once the universe cooled, a new, powerful and as yet unknown force might have bound these component particles together tightly to form electrically neutral composites.

Stealth dark matter particles should be stable — not decaying over eons, if at all, much like protons.

However, the researchers suggest the components making up stealth dark matter particles can form different unstable composites that decay shortly after their creation.

These unstable particles might have masses of about 100 billion electron-volts or more, and could be created by particle accelerators such as the Large Hadron Collider (LHC) beneath the France-Switzerland border. They could also have an electric charge and be visible to particle detectors, Vranas said.

Experiments at the LHC, or sensors designed to spot rare instances of dark matter colliding with ordinary matter, “may soon find evidence of, or rule out, this new stealth dark matter theory,” Vranas said in a statement.

If stealth dark matter exists, future research can investigate whether there are any effects it might have on the cosmos.

The scientists, the Lattice Strong Dynamics Collaboration, will detail their findings in an upcoming issue of the journal Physical Review Letters.

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Dwarf Galaxies Loom Large in Quest for Dark Matter

In its inaugural year of observations, the Dark Energy Survey has already turned up at least eight objects that look to be new satellite dwarf galaxies of the Milky Way.

These miniature galaxies — the first discovered in a decade — shine with a mere billionth of our galaxy’s brightness and each contain a million times less mass.

Astronomers believe the vast majority of material in dwarf galaxies is dark matter, a mysterious substance composing 80 percent of all matter in the universe.

Dwarf galaxies have therefore emerged as prime targets for gathering potential clues about dark matter’s composition.




Some theories suggest dark matter particles and antiparticles should produce telltale gamma rays when they collide with each other.

Accordingly, scientists used the Fermi Gamma-Ray Space Telescope to study the newfound dwarf galaxy candidates, as well as a group of dwarf galaxies already on the books.

The telescope detected no significant gamma-ray signals from either set of dwarf galaxies, however, leaving scientists still in the hunt for dark matter.

On May 15, 2015, The Kavli Foundation spoke with three astrophysicists about the continuing search for dark matter data in space and how dwarf galaxies can help us understand the evolution of our universe.

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