Physicists just debunked one of the most promising candidates for dark matter

Original article from the excellence Science Alert website.

From dark energy yesterday to dark matter today!

You probably know that just 15 percent of the known Universe is made up of matter that we can actually see. The majority of the Universe – some 85 percent of it – is made up of dark matter and dark energy – two phenomena that are currently 100 percent unknown to science, despite the best efforts of researchers worldwide.

But now, thanks to a paper authored by over 100 physicists… well, it’s still unknown, but it’s just a little less unknown than it was before, because one of the top candidates for dark matter has pretty much been debunked.

The kind of matter that makes up everything we’ve ever seen in the Universe, from tiny quarks to massive galaxies, is only 15 percent of the matter that’s actually out there. The rest is known enigmatically as dark matter, because we can’t see it and no one knows what it is, but we’re almost positive that it’s out there, unless we have to seriously rethink our understanding of the laws of gravity – the force that governs everything in the known Universe.

There are some scientists out there doing this kind of rethinking, but most agree that dark matter has to be something. They just disagree about what that something actually is. The leading contender is a class of Weakly Interacting Massive Particles, or WIMPs. But there are other possibilities with exciting names like axions, axion-like particles, and supersymmetric particles.

Now, thanks to the Fermi Large Area Telescope, the array of possibilities is starting to thin out.

Axions were first proposed in 1977 to resolve a problem in quantum chromodynamics – the theory of how quarks interact with one another. Later, when they were developing string theory over the next 10 or 20 years, they noticed some particles showing up in it that looked a lot like axions.

Physicists are famously good at naming things, so they called these exciting new particles axion-like particles, or ALPs.

It wasn’t long before they realised that axions and ALPs might also make good candidates for dark matter. When the Big Bang created all of the light and matter in the Universe, it should have also created a whole bunch of axions and ALPs – if they exist. But if they do, these particles probably would’ve congregated right where see evidence of dark matter.

Dark matter is hard to see – that’s what makes it dark matter – so to look for it, you need to think of something clever that no one has tried before. And scientists hadn’t really tried looking at gamma rays, so these researchers looked at gamma rays.

Every once in a while, you’d expect an axion or ALP to run into a bit of regular matter, which should send a gamma ray out into space with a specific energy. These gamma rays would then be visible to modern telescopes like the Fermi Large Area Telescope (LAT).

Different models of ALPs predict different numbers of them in the Universe: some models say that all of the dark matter could be ALPs, others say that they make up only a tiny fraction of it. These different models predict different amounts of gamma rays, so you can use the number and kind of gamma rays observed to test the different models of ALPs.

That’s a bunch of steps, but it’s exactly what a team of 102 scientists has done in a recent paper in Physical Review Letters.

They used six years of LAT data on the galaxy NGC 1275 (another very creative name), and checked to see if the observed gamma rays matched some popular models where ALPs make up about 5 percent of the dark matter in the Universe. If these ALP models were right, that would still leave 80 percent of the mass in the Universe unexplained. But you have to start somewhere with these things.

It looks like we’ll have to start somewhere else. The team simulated galaxies with and without the ALPs and then they checked the results of these simulations against those six years of observations. They found that the ALPs don’t seem to predict the observed gamma rays any better than the model without them.

And in science, if you have two hypotheses that perform equally well, you get rid of the one with more stuff in it. In this case, you get rid of the one with those ALPs.

There’s still a big range of possibilities to explore for LAT and for future gamma-ray telescopes. The most obvious one that the researchers mention is a model where ALPs make up all dark matter, not just 5 percent of it. But testing this model is going to take some time.

So it’s possible that in the next few years, we’ll discover what makes up all of the dark matter in the Universe. Or we’ll discover what doesn’t make it up. Either way, that’s pretty exciting.

Feature image: Artwork shows a black hole surrounded by axions (TU Wien)


NASA Detects Strange Signals From Gravitational Wave Source

Original article via Science Alert

Less than half a second after the first direct evidence of gravitational waves was recorded on 14 September 2015, a very short, faint signal was registered by NASA’s Fermi Telescope from the same region in space.

High-energy light particles called gamma rays were caught emanating from a black hole merger in the area, and the discovery will not only help physicists pinpoint the exact source of the gravitational wave – if confirmed, it has huge implications for our understanding of the fundamental physics that govern our Universe.

“Gamma-rays arising from a black hole merger would be a landmark finding because black holes are expected to merge ‘cleanly’, without producing any sort of light,” NASA explains.

First off, here’s what we know. On September 14, the Laser Interferometer Gravitational-Wave Observatory (LIGO) facilities in Washington and Louisiana picked up the first direct evidence of Einstein’s gravitational waves, traced to the merging of two black holes (called binary black holes) around 1.3 billion years ago.

The discovery was significant for two reasons, as Fiona MacDonald reported for us earlier this year:

“This event – which in itself is a big deal, seeing as no one had ever spotted a binary black hole merger before – was so massive that it significantly warped the fabric of space time, creating ripples that spread out across the Universe… finally reaching us last year.”

Now, researchers at NASA have just announced that they too picked up on something strange on September 14 – a very faint burst of gamma rays that occurred less than half a second after the gravitational waves, and in the same region of space.

Coincidence? We can’t discount it just yet, but NASA says there’s a 0.2 percent chance of these two events randomly occurring in the same place at the same time.

At the very least, the discovery – which was picked up by the Gamma-ray Burst Monitor (GBM) on NASA’s Fermi Gamma-ray Space Telescope – will help scientists figure out exactly where this black hole merger occurred 1.3 billion years ago.

As NASA explains:

“Currently, gravitational wave observatories possess relatively blurry vision. For the September event, dubbed GW150914 after the date, LIGO scientists could only trace the source to an arc of sky spanning an area of about 600 square degrees, comparable to the angular area on Earth occupied by the United States.

Assuming the GBM burst is connected to this event, the GBM localisation and Fermi’s view of Earth combine to reduce the LIGO search area by about two-thirds, to 200 square degrees.”

LIGO-mapMap of the original source of the September 14 gravitational wave event, which has now been narrowed down by two-thirds. Credit: LIGO/Axel Mellinger

But the fact that light appears to have been emitted from a black hole merger could also prompt a massive rethink of one of the most violent, high-energy events in the known Universe.

Why? Well, simply put, gas is needed in order to generate light, and there should be no gas around two soon-to-merge black holes, because it should have been swallowed up by one of them long before the pair collide.

There are now two possibilities. The first is the gamma ray burst really was a coincidence and wasn’t related to the GW150914 black hole merger that produced the September 14 gravitational waves. The second is that black hole mergers really can produce an observable gamma-ray emission, and that means we’re going to have to rethink the laws that govern what black holes can swallow and when.

“This is a tantalising discovery with a low chance of being a false alarm, but before we can start rewriting the textbooks we’ll need to see more bursts associated with gravitational waves from black hole mergers,” one of the the GBM team, Valerie Connaughton, told Francis Reddy at

We’re now going to need more data to figure out which, but gas particles escaping the pull of black holes? That would be pretty damn amazing.

The results have been published in The Astrophysical Journal.

The Universe is Expanding Faster Than We Thought

From the good people at Science Alert

The Universe is expanding faster than the laws of physics can explain, new measurements reveal – ScienceAlert<img src=”” style=”display:none” height=”1″ width=”1″ alt=””/><img height=”1″ width=”1″ alt=”” style=”display:none” src=””/>

The Universe is expanding faster than the laws of physics can explain, new measurements reveal

Time for some new physics?

The Universe is expanding faster than the laws of physics can explain, new measurements reveal – ScienceAlert<img src=”” style=”display:none” height=”1″ width=”1″ alt=””/><img height=”1″ width=”1″ alt=”” style=”display:none” src=””/>

The most precise measurement ever made of the current rate of expansion of the Universe has been achieved by physicists in the US, and there’s a problem: the Universe is expanding 8 percent faster than our current laws of physics can explain.

If confirmed by independent tests, this new measurement will force us to rethink how dark matter and dark energy have been influencing the evolution of the Universe for the past 13.8 billion years, and that means something in the standard model of particle physics has to change.

The Universe is expanding faster than the laws of physics can explain, new measurements reveal – ScienceAlert<img src=”” style=”display:none” height=”1″ width=”1″ alt=””/><img height=”1″ width=”1″ alt=”” style=”display:none” src=””/>

“I think that there is something in the standard cosmological model that we don’t understand,” lead researcher Adam Riess from Johns Hopkins University, who also co-discovered dark energy back in 1998, told Davide Castelvecchi at Nature.

So… wtf just happened? Well, right now, physicists explain the gradual expansion of the Universe – which has been in effect since the Big Bang – by the presence of dark matter and dark energy.

While invisible dark matter is thought to make up 27 percent of the Universe, and visible matter a measly 5 percent, dark energy is estimated to make up a whopping 68 percent of the known Universe, and the way all three interact could explain why everything has been expanding since the beginning of time.

According to the accepted model of cosmology, the biggest influence on the evolution of the Universe is the competition between dark matter and dark energy. While the gravitational pull of dark matter appears to be slowing down the expansion of the Universe, dark energy seems to be tugging it in the opposite direction to make it accelerate.

Astrophysicists were able to figure all this out thanks to measurements of radiation left over from the Big Bang, which we can now observe as the Cosmic Microwave Background, or CMB.

Earlier observations of the CMB made by Riess and other astrophysicists around the world have suggested that the pull of dark energy on the Universe has remained constant since the Big Bang, Castelvecchi reports.

The Universe is expanding faster than the laws of physics can explain, new measurements reveal – ScienceAlert<img src=”” style=”display:none” height=”1″ width=”1″ alt=””/><img height=”1″ width=”1″ alt=”” style=”display:none” src=””/>

This hypothesis was backed up by the most comprehensive analysis of the CMB, performed recently by the European Space Agency’s Planck Observatory, and scientists have since used Planck’s measurements to estimate the rate of expansion at any point in the Universe’s history.

cmbImage of the cosmic microwave background. Credit: NASA

“For years, those predictions have disagreed with direct measurements of the current rate of cosmic expansion – also known as the Hubble constant,” says Castelvecchi. “But until now the error margins in this constant were large enough that the disagreement could be ignored.”

Now Riess and his colleagues have found another way to measure the rate of expansion – the brightness of certain types of celestial objects, such as stars and supernovae, known as ‘standard candles’.

As Kelly Dickerson explains for, standard candles are thought to emit the exact same level of brightness, which means physicists can use them as markers to measure how fast the Universe is expanding away from us.

Riess’s team analysed 18 standard candles using hindreds of hours of data from the Hubble Space Telescope, and calculated that the speed of expansion is about 8 percent faster than the Planck’s measurements predicted.

“If this new measurement is accurate – and our maps of the CMB are also accurate – then something about our fundamental understanding of the Universe is wrong,” says Dickerson.

These results, which have been posted to pre-print website and are awaiting peer-review, have the potential of “becoming transformational in cosmology”, cosmologist Kevork Abazajian from the University of California, who was not involved in the study, told Nature.  

We’re going to have to sit tight and wait for these results to be independently confirmed or disproved, but we’ve been hearing pretty often recently of things happening out in our Universe that challenge our current laws of physics, so something’s probably gonna have to give eventually.

One thing’s for sure – it’s an exciting time to be a physicist.

Photo:  M101 Credit NASA/JPL-Caltech<img src=”” style=”display:none” height=”1″ width=”1″ alt=””/><img height=”1″ width=”1″ alt=”” style=”display:none” src=””/>

Stephen Hawking Launches $100Million “Starshot” project to find Alien life

From Scientific American:

$100-Million Plan Will Send Probes to the Nearest Star

Funded by Russian entrepreneur Yuri Milner and with the blessing of Stephen Hawking, Breakthrough Starshot aims to send probes to Alpha Centauri in a generation

For Yuri Milner, the Russian Internet entrepreneur and billionaire philanthropist who funds the world’s richest science prizes and searches for extraterrestrial intelligence, the sky is not the limit—and neither is the solar system. Flanked by physicist Stephen Hawking and other high-profile supporters today in New York, Milner announced his most ambitious investment yet: $100 million toward a research program to send robotic probes to nearby stars within a generation.

“The human story is one of great leaps,” Milner said in a statement released shortly before the announcement. “55 years ago today, Yuri Gagarin became the first human in space. Today, we are preparing for the next great leap—to the stars.”

“Breakthrough Starshot,” the program Milner is backing, intends to squeeze all the key components of a robotic probe—cameras, sensors, maneuvering thrusters and communications equipment—into tiny gram-scale “nanocrafts.” These would be small enough to boost to enormous speeds using other technology the program plans to help develop, including a ground-based kilometer-scale laser array capable of beaming 100-gigawatt laser pulses through the atmosphere for a few minutes at a time, and atoms-thin, meter-wide “light sails” to ride those beams to other stars. Each pinging photon of light would impart a slight momentum to the sail and its cargo; in the microgravity vacuum of space, the torrent of photons unleashed by a gigawatt-class laser would rapidly push a nanocraft to relativistic speeds.

“Without new methods of propulsion we simply cannot get very far,” Hawking said at the announcement. “Light is the most pragmatic technology available.”

Deployed by the thousands from a mothership launched into Earth orbit, each nanocraft would unfurl a sail and catch a laser pulse to accelerate to 20 percent the speed of light—some 60,000 kilometers per second. Using a sophisticated adaptive-optics system of deformable mirrors to keep each pulse coherent and sharp against the blurring effects of the atmosphere, the laser array would boost perhaps one orbiting nanocraft per day. Each laser pulse would contain as much power as that produced by a space shuttle rocketing into orbit.

The array would need to be built at a dry, high-altitude location in the Southern Hemisphere, perhaps on a peak in Chile, South Africa or even Antarctica—somewhere within sight of Breakthrough Starshot’s primary targets: the twin stars of Alpha Centauri, which at 4.37 light years away make up the nearest neighboring star system to our own. NASA has already sent five spacecraft on trajectories taking them beyond our solar system, though even the fastest of these would require more than 30,000 years to reach Alpha Centauri. The nanocrafts would make that same interstellar crossing in just 20 years. With no onboard ability to decelerate, they would briefly gather data about any planets in the Alpha Centauri system and beam it back toward Earth before plunging deeper into interstellar darkness and out of communication range.

“If this mission comes to fruition it will tell us as much about ourselves as about Alpha Centauri,” Milner said at the press conference.

Breakthrough Starshot is the latest of Milner’s Breakthrough Initiatives, a multidisciplinary collection of projects marshaling private funds to address existential questions about life in the universe. Last year, also with Hawking, he announced the $100 million, 10-year “Breakthrough Listen” initiative to search more than a million stars and a hundred galaxies for signals from alien civilizations, as well as an accompanying $1 million “Breakthrough Message” initiative to compose potential cosmic communiqués to broadcast to any attentive extraterrestrials. Like Breakthrough Starshot, which involves the most sizeable lump sum ever dedicated purely to achieving interstellar flight, these other initiatives offered similar financial sea changes for their respective fields—which due to their extremely speculative nature have long languished in the hinterlands of federal science funding.

Big Plans, Small Spacecraft

Serious planning for the project began about a year ago, when Milner consulted experts to consider options for practical interstellar travel. One was Avi Loeb, an astrophysicist at Harvard University and new chairman of Breakthrough Starshot’s advisory board who has a reputation for performing groundbreaking work on unconventional research topics.

Loeb and his fellow consultants noted that we already routinely accelerate subatomic particles near-light speed in modern particle physics experiments, and that the smaller a spacecraft is, the more likely it can be made to travel at an extreme velocity. “Strip an iPhone from its case and interface, and the electronics—including the camera and the communications device—weigh on the order a gram,” Loeb says. “That’s almost everything you need for a nanocraft, and we practically have it right now thanks to the ongoing miniaturization of electronics.”

After evaluating and dismissing propulsion options as exotic as rockets fueled by antimatter annihilation or nuclear fusion reactions, the consultants narrowed their considerations to laser-powered light sails, a concept dating back to the 1960s. They focused on the recent work of Philip Lubin, a physicist at the University of California, Santa Barbara, who was just completing a “roadmap” for developing minuscule, laser-powered interstellar spacecraft as part of a modest NASA-funded study. With minor tweaks, that roadmap offered a notional template for Breakthrough Starshot, and Lubin is now one of the project’s key scientists.

“There are two axes to the problem of interstellar flight,” Lubin says. “Things like antimatter or fusion rockets are all on the ‘real’ axis. The known laws of physics tell us they are realistic solutions, even if we don’t know how to realize them. Things like warp drives and wormholes are on the ‘imaginary’ axis—these are what I would call fictional solutions, because no one knows how to do them.” The laser propulsion concept Lubin detailed in his roadmap rates high on his ‘real’ axis, he says, because “it is both realistic and realizable.”

Lubin’s roadmap laid out myriad obstacles that any laser-propelled interstellar mission would have to overcome, such as linking many smaller lasers into a kilometer-scale array and engineering lightweight, gossamer-thin sails strong enough to endure the array’s gigawatt-scale pulses, as well as persuading policymakers to allow the construction of a laser system that could in principle be used as a weapon. The probes will also need to transmit observations back to Earth using onboard lasers with just a few watts of power—a problem potentially solvable by using the giant earthbound laser array as a receiver. But the biggest obstacle of all was simply a matter of cost: At an estimated present-day price of approximately $10 per watt of laser power, building and operating Breakthrough Starshot’s 100-gigawatt array today could cost as much as $1 trillion.

But as steep as that sounds now, the market prices of just 10 years ago would have rendered it a hundred times more expensive. Driven by demand in consumer high-speed telecommunications systems as well as defense-related projects, the cost of critical technologies for a gigantic laser array are now decreasing by approximately a factor of two every 18 months, Lubin says. Those exponential rates of change suggest that in 10 years a giant laser array’s per-watt cost would drop from $10 to 10 cents. “Breakthrough Starshot is really about choosing core technologies to scale up massively, and looking at what prevents or enables that scaling,” Lubin says. “If things are static for the next 30 years in terms of cost per watt, we will be in big trouble.”

According to Loeb, however, another obstacle is a more subtle and social phenomenon: the “giggle factor,” or the tendency for far-out concepts to be laughed off. “Any revolution in science or technology has an initial phase where people laugh at it,” Loeb says. “Sometimes the laughter is inspired by valid criticisms of an argument, but can also be because something appears very different and strange…What is certain is that the mainstream scientific community that works on research you are not supposed to laugh about—research that has a giggle factor of zero, let’s say—keeps making major mistakes in giggling about the wrong things.”

“We are serious people,” Loeb continues. “We will find whether this project is doable or not, and if it is not, we will admit that and move on.”

Centauri Dreams

Following Lubin’s tweaked roadmap, most of Milner’s $100 million is meant to fund research grants to develop solutions to about 20 major technical obstacles identified by the project. Those solutions would then feed into a prototype system that could perhaps be built for a few hundred million dollars more. Provided that photonics and electronics technologies continue their trends of plummeting costs and soaring performance, assembling a fully functional system would require funding on the same scale as the world’s current multibillion-dollar science projects such as the Large Hadron Collider and the James Webb Space Telescope—funding that Milner alone could not provide. Governments would be one possible sponsor; collectives of billionaire philanthropists would be another. Along with Milner and Hawking, the third member of Breakthrough Starshot’s board of directors is Mark Zuckerberg, the wealthy founder and CEO of Facebook.

“This must be viewed as a collective effort, because that is the only way this can be done,” Milner says. “If we can do this within our lifetimes as we hope, that is pretty exciting, but if not, we will pass it to the next generation—not as an idea, but as a developed roadmap and technology. We are not hundreds of years away from this—only dozens… This $100 million is meant to last for the next few years, to focus on every single one of the potential dealbreakers we have found, to see how far we an push and if we hit any roadblocks.”

Besides the present lack of hardware and full-scale funding, one more key thing is missing from Breakthrough Starshot’s plans: There are as yet no confirmed planetary targets in Alpha Centauri. In 2012 a team of European astronomers announced their discovery of an Earth-sized planet in a three-day orbit around one of the system’s two stars, but further investigation cast serious doubts about those claims. According to Breakthrough Initiatives’ chairman Pete Worden, the former director of NASA’s Ames Research Center and current director of Breakthrough Starshot, the organization is also planning a second, related initiative to build new ground-based instruments and maybe even a small space telescope to search for and study Alpha Centauri’s possible planets. Such instruments and telescopes could be turned to other nearby stars, too, possibly revealing additional targets for interstellar voyages. Ultimately, nanocrafts in their wispy millions could fan out on photons to explore many more stars, transforming what we do and know on galactic scales.

In addition to making practical interstellar flight a reality, Worden says, the Starshot project could also be transformative for other applications closer to home. Starshot’s final laser array could prove useful for detecting and characterizing potentially threatening near-Earth asteroids, and its many deformable mirrors could be repurposed to make the array a massive telescope for gathering starlight rather than creating laser beams. The lasers could also power nanocrafts on rapid flybys of all the solar system’s planets, sending probes to Mars in a few hours or Pluto in days for a few hundred thousand dollars per shot.

“Once we launch these things and deliver close-up images of a planet around another star, that begins to define humanity as a whole and humanity’s future,” Worden says. “This is something that, if it works, changes how we think about ourselves as a species and as a planet.”

Already, the planning for Breakthrough Starshot is changing how members of the Breakthrough Initiatives organization view its other related projects, such as Breakthrough Listen, the effort to tune in to deliberate or inadvertent transmissions from cosmic civilizations. If we can seek out potentially habitable planets in other star systems and use lasers to send flotillas of miniaturized spacecraft to investigate them, there is no reason to assume we are the only ones in the Milky Way doing so. The spacecraft themselves would be essentially invisible: A gram-sized interstellar probe striking the Earth’s upper atmosphere at 20 percent the speed of light would release roughly a kiloton of energy, indistinguishable from airbursts produced by meter-scale space rocks that regularly pepper our planet at a rate of about once per year. However, were it to stream into one of Earth’s more capable ground-based observatories, the light from the lasers propelling such probes could conceivably be detected. Most of Breakthrough Listen’s efforts revolve around seeking signs of chatty aliens conversing via radio waves, but the project also funds searches for any glints of laser light from interstellar space.

In remarks prepared ahead of Tuesday’s news conference, Stephen Hawking explained his support for the project as less about science and more about survival. “Earth is a wonderful place, but it might not last forever,” Hawking says. “Sooner or later, we must look to the stars. Breakthrough Starshot is a very exciting first step on that journey.”

Loeb agrees. “At some point, we must find alternatives to living on Earth, and as with any big journey, this has to start with a first small step—you can’t just give up,” he says. “Remember what Oscar Wilde said—‘We are all in the gutter, but some of us are looking at the stars.’ When I look up at the stars at night, they seem like lights in a giant ship sailing through space. And each time I ask myself the same question: Are there other passengers riding in this giant ship, near those lights? If this project comes to fruition, we could visit them and find out.”

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