BLOGS

An artist's impression of the cosmic web (Volker Springel/Max Planck Institute for Astrophysics/et al)

Topics: Astronomy, Astrophysics, Cosmology, Dark Matter, Einstein, Research

Q: Why does science seem to always change its mind?

A: Because, the enterprise of science is about discovery, and a lot of discoveries happen when you have better instrumentation, apply The Scientific Method and rigorous peer review. (See the quote below by economist John Maynard Keynes)

When science tried to fit the universe into a mechanistic viewpoint, it coined something called the luminiferous aether. Albert Michelson and Edward Morley built what we now know as an interferometer to measure its existence, and ended up measuring the speed of light. It earned them the Nobel Prize. Theirs were the shoulders on which Einstein built his Theory of Relativity, along with Newton, Faraday and others. Knowing better, science changes its mind.

“When the facts change, I change my mind - what do you do, sir?”
― John Maynard Keynes

There might not be a mysterious 'dark' force accelerating the expansion of the Universe after all. The truth could be much stranger – bubbles of space where time passes at drastically different rates.

The passage of time isn't as constant as our experience with it suggests. Areas of higher gravity experience a slower pace of time compared with areas where gravity is weaker, a fact that could have some pretty major implications on how we compare rates of cosmic expansion according to a recently developed model called timescape cosmology.

Discrepancies in how fast time passes in different regions of the Universe could add up to billions of years, giving some places more time to expand than others. When we look at distant objects through these time-warping bubbles, it could create the illusion that the expansion of the Universe is accelerating.

Two new studies have analyzed more than 1,500 supernovae to investigate how likely the concept could be – and found that the timescape model might be a better fit for observations than our current best model.

The standard model of cosmology does a pretty good job of explaining the Universe – provided we fudge the numbers a bit. There doesn't seem to be enough mass to account for the gravitational effects we observe, so we invented an invisible placeholder called dark matter.

There also seems to be a strange force that counteracts gravity, pushing the cosmos to expand at accelerating rates. We don't know what it is yet, so in the same spirit we dubbed it dark energy. All of this comes together, along with ordinary matter, to form what we call the lambda cold dark matter (ΛCDM) model.

The problem is that this model uses a simplified equation that assumes the whole Universe is smooth, and expands at the same speed everywhere. But it's far from smooth out there: we see a colossal cosmic web, crisscrossed by filaments of galaxies separated by vast voids emptier than we can comprehend.

Timescape cosmology takes that 'lumpiness' into account. More matter means stronger gravity, which means slower time – in fact, an atomic clock located in a galaxy could tick up to a third slower than the same clock in the middle of a void.

When you stretch that over the huge lifespan of the Universe, billions more years may have passed in the voids than in the matter-dense areas. A mind-boggling implication of that is that it no longer makes sense to say that the Universe has a single unified age of 13.8 billion years. Instead, different regions would have different ages.

And since so much more time has passed in the voids, more cosmological expansion has taken place there. Therefore, if you look at an object on the far side of a void, it would appear to be moving away from you much faster than something on this side of the void. Over time, these voids take up a larger proportion of the Universe, creating the illusion of an accelerating expansion, without needing to conjure up any dark energy.

Dark Energy May Not Exist: Something Stranger Might Explain The Universe, Hinah Shah, Shining Science

Artist's impression of a microlensing event caused by a black hole observed from Earth toward the Large Magellanic Cloud. The light of a background star located in the LMC is bent by a putative primordial black hole (lens) in the Galactic halo and magnified when observed from the Earth. Microlensing causes very characteristic variation of brightness of the background star, enabling the determination of the lens's mass and distance. Credit: J. Skowron / OGLE. Background image of the Large Magellanic Cloud: generated with bsrender written by Kevin Loch, using the ESA/Gaia database

Topics: Astronomy, Astrophysics, Black Holes, Dark Matter

The gravitational wave detectors LIGO and Virgo have detected a population of massive black holes whose origin is one of the biggest mysteries in modern astronomy. According to one hypothesis, these objects may have formed in the very early universe and may include dark matter, a mysterious substance filling the universe.

A team of scientists from the OGLE (Optical Gravitational Lensing Experiment) survey from the Astronomical Observatory of the University of Warsaw have announced the results of nearly 20-year-long observations indicating that such massive black holes may comprise at most a few percent of dark matter. Another explanation, therefore, is needed for gravitational wave sources. The results of the research were published in a study in Nature and a study in The Astrophysical Journal Supplement Series.

Various astronomical observations indicate that ordinary matter, which we can see or touch, comprises only 5% of the total mass and energy budget of the universe. In the Milky Way, for every 1 kg of ordinary matter in stars, there is 15 kg of dark matter, which does not emit any light and interacts only by means of its gravitational pull.

"The nature of dark matter remains a mystery. Most scientists think it is composed of unknown elementary particles," says Dr. Przemek Mr.óz from the Astronomical Observatory, University of Warsaw, the lead author of both articles. "Unfortunately, despite decades of efforts, no experiment (including experiments carried out with the Large Hadron Collider) has found new particles that could be responsible for dark matter."

New research challenges black holes as dark matter explanation, University of Warsaw, Phys.org.

Source: Same source for the Dark Matter definition below.

Topics: Astronomy, Astrophysics, Cosmology, Dark Matter, Einstein, General Relativity

Note: Your "secret decoder ring" for reading the Abstract.

Dark matter: It makes up about 85% of the universe, is invisible, and doesn't interact with matter except for gravitational effects. See: Center for Astrophysics, Harvard

"Tachyonic": Of, or referring to tachyons, (Greek for swift) theoretical particles that already travel faster-than-light and backward in time. Their rest mass, m₀ⁱ, is assumed to be imaginary. As it loses energy, it's assumed to become infinitely fast, so you can see why it's a favorite science fiction trope, along with dark matter, literally tableau rasas.

ΛCDM assumes that the universe is composed of photons, neutrinos, ordinary matter (baryons, electrons), and cold (non-relativistic) dark matter, which only interacts gravitationally, plus "dark energy," which is responsible for the observed acceleration in the Hubble expansion. Source: Goddard Spaceflight Center: Lambda

H₀ defines the Hubble constant, or, the rate at which the universe is expanding, determined by Hubble in the way back year of 1929 to be 500 km/s/Mpc. I'm going to defer to Wikipedia for this one.

km/s/Mpc = kilometers/second/megaparsec. Megaparsec is 1 million parsecs = 3,260,000 light years, or 3.26 x 10⁶ light years.

t₀ = the present age of the universe, t₀ = 2tH/3, where "tH" is the Hubble time. t₀ is roughly 13.7 × 10⁹ years, or 4.32 × 10¹⁷ seconds.

Gyr = giga years, or 1 billion years = 1 x 10⁹ years (a lot).

Abstract

An open or hyperbolic Friedmann-Robertson-Walker spacetime dominated by tachyonic dark matter can exhibit an “inflected” expansion—initially decelerating, later accelerating—similar but not identical to that of now-standard ΛCDM models dominated by dark energy. The features of the tachyonic model can be extracted by fitting the redshift-distance relation of the model to data obtained by treating Type Ia supernovae as standard candles. Here such a model is fitted to samples of 186 and 1048 Type Ia supernovae from the literature. The fits yield values of H₀ = (66.6±1.5) km/s/Mpc and H₀ = (69.6±0.4) km/s/Mpc, respectively, for the current-time Hubble parameter, and t₀ = (8.35 ± 0.68) Gyr and t₀ = (8.15 ± 0.36) Gyr, respectively, for the comoving-time age of the Universe. Tests of the model against other observations will be undertaken in subsequent works.

Subject headings: cosmology, dark matter, tachyons, distance-redshift relation, supernovae

Testing Tachyon-Dominated Cosmology with Type Ia Supernovae, Samuel H. Kramer, Ian H. Redmount, Physics arXiv

Topics: Astronomy, Astrophysics, Dark Matter, Research, Theoretical Physics

Dark matter, composed of particles that do not reflect, emit, or absorb light, is predicted to make up most of the matter in the universe. However, its lack of interactions with light prevents its direct detection using conventional experimental methods.

Physicists have been trying to devise alternative methods to detect and study dark matter for decades, yet many questions about its nature and its presence in our galaxy remain unanswered. Pulsar Timing Array (PTA) experiments have been trying to probe the presence of so-called ultralight dark matter particles by examining the timing of an ensemble of galactic millisecond radio pulsars (i.e., celestial objects that emit regular millisecond-long radio wave pulses).

The European Pulsar Timing Array, a multinational team of researchers based at different institutes that are using 6 radio-telescopes across Europe to observe specific pulsars, recently analyzed the second wave of data they collected. Their paper, published in Physical Review Letters, sets more stringent constraints on the presence of ultralight dark matter in the Milky Way.

"This paper was basically the result of my first Ph.D. project," Clemente Smarra, co-author of the paper, told Phys.org. "The idea arose when I asked my supervisor if I could carry out research focusing on gravitational wave science, but from a particle physics perspective. The main aim of the project was to constrain the presence of the so-called ultralight dark matter in our galaxy."

Ultralight dark matter is a hypothetical dark matter candidate, made up of very light particles that could potentially address long-standing mysteries in the field of astrophysics. The recent study by Smarra and his colleagues was aimed at probing the possible presence of this type of dark matter in our galaxy via data collected by the European Pulsar Timing Array.

"We were inspired by previous efforts in this field, especially by the work of Porayko and her collaborators," Smarra said. "Thanks to the longer duration and the improved precision of our dataset, we were able to put more stringent constraints on the presence of ultralight dark matter in the Milky Way,"

The recent paper by the European Pulsar Timing Array makes different assumptions than those made by other studies carried out in the past. Instead of probing interactions between dark matter and ordinary matter, it assumes that these interactions only occur via gravitational effects.

"We assumed that dark matter interacts with ordinary matter only through gravitational interaction," Smarra explained. "This is a rather robust claim: in fact, the only sure thing we know about dark matter is that it interacts gravitationally. In a few words, dark matter produces potential wells in which pulsar radio beams travel. But the depth of these wells is periodic in time; therefore, the travel time of the radio beams from pulsars to the Earth changes with a distinctive periodicity as well."

New constraints on the presence of ultralight dark matter in the Milky Way, Ingrid Fadelli, Phys.org.

Magnet row of the ALPS experiment in the HERA tunnel: In this part of the magnets, intense laser light is reflected back and forth, from which axions are supposed to form. Credit: DESY, Marta Maye

Topics: Dark Matter, Materials Science, Particle Physics, Quantum Mechanics

The ALPS (Any Light Particle Search) experiment, which stretches a total length of 250 meters, is looking for a particularly light type of new elementary particle. The international research team wants to search for these so-called axions or axion-like particles using twenty-four recycled superconducting magnets from the HERA accelerator, an intense laser beam, precision interferometry, and highly sensitive detectors.

Such particles are believed to react only extremely weakly with known kinds of matter, which means they cannot be detected in experiments using accelerators. ALPS is therefore resorting to an entirely different principle to detect them: in a strong magnetic field, photons—i.e., particles of light—could be transformed into these mysterious elementary particles and back into [light] again.

"The idea for an experiment like ALPS has been around for over 30 years. By using components and the infrastructure of the former HERA accelerator, together with state-of-the-art technologies, we are now able to realize ALPS II in an international collaboration for the first time," says Beate Heinemann, Director of Particle Physics at DESY.

Helmut Dosch, Chairman of DESY's Board of Directors, adds, "DESY has set itself the task of decoding matter in all its different forms. So ALPS II fits our research strategy perfectly, and perhaps it will push open the door to dark matter."

The ALPS team sends a high-intensity laser beam along a device called an optical resonator in a vacuum tube, approximately 120 meters in length, in which the beam is reflected backward and forwards and is enclosed by twelve HERA magnets arranged in a straight line. If a photon were to turn into an axion in the strong magnetic field, that axion could pass through the opaque wall at the end of the line of magnets.

Once through the wall, it would enter another magnetic track almost identical to the first. Here, the [axion] could then change back into a photon, which would be captured by the detector at the end. A second optical resonator is set up here to increase the probability of an [axion[ turning back into a photon by a factor of 10,000.

This means if [light] does arrive behind the wall, it must have been an axion in between. "However, despite all our technical tricks, the probability of a photon turning into an axion and back again is very small," says DESY's Axel Lindner, project leader and spokesperson of the ALPS collaboration, "like throwing 33 dice and them all coming up the same."

In order for the experiment to actually work, the researchers had to tweak all the different components of the apparatus to maximum performance. The light detector is so sensitive that it can detect a single photon per day. The precision of the system of mirrors for the light is also record-breaking: the distance between the mirrors must remain constant to within a fraction of an atomic diameter relative to the wavelength of the laser.

World's most sensitive model-independent experiment starts searching for dark matter, Deutsches Elektronen-Synchrotron, Phys.org.

Multiple images of a background image created by gravitational lensing can be seen in the system HS 0810+2554. Credit: Hubble Space Telescope / NASA / ESA

Topics: Astronomy, Astrophysics, Dark Matter, Einstein, General Relativity

Physicists believe most of the matter in the universe is made up of an invisible substance that we only know about by its indirect effects on the stars and galaxies we can see.

We're not crazy! Without this "dark matter," the universe as we see it would make no sense.

But the nature of dark matter is a longstanding puzzle. However, a new study by Alfred Amruth at the University of Hong Kong and colleagues, published in Nature Astronomy, uses light's gravitational bending to bring us a step closer to understanding.

Invisible but omnipresent

We think dark matter exists because we can see its gravity's effects on galaxies' behavior. Specifically, dark matter seems to make up about 85% of the universe's mass, and most of the distant galaxies we can see appear to be surrounded by a halo of the mystery substance.

But it's called dark matter because it doesn't give off light or absorb or reflect it, which makes it incredibly difficult to detect.

So what is this stuff? We think it must be some kind of unknown fundamental particle, but beyond that, we're not sure. All attempts to detect dark matter particles in laboratory experiments have failed, and physicists have debated its nature for decades.

Scientists have proposed two leading hypothetical candidates for dark matter: relatively heavy characters called weakly interacting massive particles (or WIMPs) and extremely lightweight particles called axions. Theoretically, WIMPs behave like discrete particles, while axions behave more like waves due to quantum interference.

It has been difficult to distinguish between these two possibilities—but now light bent around distant galaxies has offered a clue.

New look at 'Einstein rings' around distant galaxies just got us closer to solving the dark matter debate, Rossana Ruggeri, Phys.org.

Rotation curve of the typical spiral galaxy M 33 (yellow and blue points with error bars) and the predicted one from the distribution of the visible matter (white line). The discrepancy between the two curves is accounted for by adding a dark matter halo surrounding the galaxy. Credit: Wikipedia

Topics: Astronomy, Astrophysics, Cosmology, Dark Matter

Although dark matter is central to the standard cosmological model, it's not without issues. There continue to be nagging mysteries about the stuff, not the least of which is the fact that scientists have found no direct particle evidence of it.

Despite numerous searches, we have yet to detect dark matter particles. Some astronomers favor an alternative, such as modified Newtonian dynamics (MoND) or the modified gravity model. And a new study of galactic rotation seems to support them.

The idea of MoND was inspired by galactic rotation. Most of the visible matter in a galaxy is clustered in the middle, so you'd expect that stars closer to the center would have faster orbital speeds than stars farther away, similar to the planets of our solar system. We observe that stars in a galaxy all rotate at about the same speed. The rotation curve is essentially flat rather than dropping off. The dark matter solution is that a halo of invisible matter surrounds galaxies, but in 1983 Mordehai Milgrom argued that our gravitational model must be wrong.

At interstellar distances, the gravitational attraction between stars is essentially Newtonian. So rather than modifying general relativity, Milgrom proposed modifying Newton's universal law of gravity. He argued that rather than the force of attraction as a pure inverse square relation, gravity has a small remnant pull regardless of distance. This remnant is only about ten trillionths of a G, but it's enough to explain galactic rotation curves.

New measurements of galaxy rotation lean toward modified gravity as an explanation for dark matter, Brian Koberlein, Universe Today/Phys.org.

Illustration of the FASER experiment. Image Credit: FASER/CERN.

Topics: CERN, Dark Matter, High Energy Physics, Neutrinos, Particle Physics

Neutrinos are ubiquitous and notorious. Billions are passing through you at this moment. Occasionally described as a “ghost of a particle,” neutrinos are nearly massless, thereby making them extremely difficult to detect experimentally (“Neutrino,” meaning “little neutral one” in Italian, was first used by Enrico Fermi in the early 1930s). Neutrinos were first confirmed in 1956 (thanks to a nearby nuclear reactor), and they’ve since been detected from different sources, including the Sun and cosmic rays, but not yet in a particle collider. Their elusiveness has been the source of much intrigue (and, of course, research funding) within the particle physics community since.

What else makes them so curious? Neutrinos come in three flavors — electron neutrino, muon neutrino, and tau neutrino — and may switch between them through the process of oscillation. Neutrino oscillations have been experimentally confirmed only in the past decade at the Super-K Detector in Japan (physicists Takaaki Kajita and Arthur B. McDonald shared the 2015 Nobel Prize in Physics for it). This discovery signified an important direction in the search for physics beyond the Standard Model because the longstanding theory does not explain neutrino oscillations and describes them as completely massless particles. Something isn’t quite adding up.

Enter: FASER. Initially proposed in 2018, the ForwArd Search ExpeRiment (FASER) is CERN’s newest experiment poised to detect neutrinos, potentially up to 1300 electron neutrinos, 20,000 muon neutrinos, and 20 tau neutrinos. Constructed in an unused service tunnel located about 500 meters from an Atlas experiment interaction point, FASER and its corresponding sub-detector, FASERν, have been designed to probe interactions of high-energy neutrinos (predicted to be between 600 GeV and 1 TeV).

FASER Poised to Further Our Understanding of Neutrinos, Dark Matter, Hannah Pell, Physics Central Buzz Blog

Clocking dark matter: optical clocks join the hunt for dark matter. (Courtesy: N Hanacek/NIST)

Topics: Dark Matter, Modern Physics, Quantum Mechanics

An optical clock has been used to set new constraints on a proposed theory of dark matter. Researchers including Jun Ye at JILA at the University of Colorado, Boulder, and Andrei Derevianko at the University of Nevada, Reno, explored how the coupling between regular matter and “ultralight” dark matter particles could be detected using the clock in conjunction with an ultra-stable optical cavity. With future upgrades to the performance of optical clocks, their approach could become an important tool in the search for dark matter.

Although it appears to account for about 85% of the matter in the universe, physicists know very little about dark matter. Most theoretical and experimental work so far has been focussed on hypothetical dark-matter particles, including WIMPS and axions, which have relatively large masses.  Alternatively, some physicists have proposed the existence of “ultralight” dark matter particles with extremely small masses that span many orders of magnitude (10⁻¹⁶–10⁻²¹ eV/c²).

According to the laws of quantum mechanics, the very smallest of these particles would have huge wavelengths, comparable to the sizes of entire dwarf galaxies – meaning they would behave like classical fields on scales we can easily measure.

Optical clock sets new constraints on dark matter, Sam Jarman, Physics World