Ask Ethan #49: Do the cosmic unknowns cast doubt on the Big Bang? — Starts With A Bang! — Medium

Ask Ethan #49: Do the cosmic unknowns cast doubt on the Big Bang? — Starts With A Bang! — Medium:  ...all isotropic, homogeneous spacetimes (that is, solutions to GR that are roughly the same at all locations in space and in all directions) must either have expanding or contracting space...

All of this — the whole story I outlined above — would be true regardless of what else is actually in your Universe. The only things that change due to dark matter and dark energy are the following...

Black Holes Aren’t Black After All, Say Theoretical Physicists — The Physics arXiv Blog — Medium

Black Holes Aren’t Black After All, Say Theoretical Physicists — The Physics arXiv Blog — Medium: Stephen Hawking proposed a potential solution earlier this year. His idea is that gravitational collapse can never continue beyond the so-called event horizon of a black hole beyond which information is lost. Gravitational collapse would approach the boundary but never go beyond it...

Cosmologists Prove Negative Mass Can Exist In Our Universe — The Physics arXiv Blog — Medium

Cosmologists Prove Negative Mass Can Exist In Our Universe — The Physics arXiv Blog — Medium: Today, Saoussen Mbarek and Manu Paranjape at the Université de Montréal in Canada say they’ve found a solution to Einstein’s theory of general relativity that allows negative mass without breaking any essential assumptions. Their approach means that negative mass can exist in our universe provided there is a reasonable mechanism for producing it, perhaps in pairs of positive and negative mass particles in the early universe...

The crucial breakthrough by Mbarek and Paranjape is to show that negative mass can produce a reasonable Schwarzschild solution without violating the energy condition. Their approach is to think of negative mass not as a solid object, but as a perfect fluid, an otherwise common approach in relativity...

First Evidence Of A Correction To The Speed of Light — The Physics arXiv Blog — Medium

First Evidence Of A Correction To The Speed of Light — The Physics arXiv Blog — Medium: Because all previous speed-of-light calculations have relied only on general relativity, they do not take into account the tiny effects of quantum mechanics. But these effects are significant over such long distances and through such a large mass as the Milky Way, says Franson...

Franson’s idea is that the gravitational potential must influence the electron-positron pair because they have mass. “Roughly speaking, the gravitational potential changes the energy of a virtual electron-positron pair, which in turn produces a small change in the energy of a photon,” he says. “This results in a small correction to the angular frequency of a photon and thus its velocity.”

New test may provide 'smoking gun' for modified gravity

New test may provide 'smoking gun' for modified gravity: Now in a new paper published in Physical Review Letters, Wojciech A. Hellwing, et al., have proposed a new test of modified gravity that is based on measuring the tendency of well-separated galaxies to approach each other. This movement is called the galaxy pairwise velocity.
The physicists show that the galaxy pairwise velocity distribution of many galaxies with a wide range of masses is expected to deviate from the predictions of general relativity by significant amounts: between 5 and 10 standard deviations or higher, depending on the model. Due to these large deviations, this proposed test could potentially offer the strongest evidence in support of modified gravity to date.

If Spacetime Were a Superfluid, Would It Unify Physics—or Is the Theory All Wet? - Scientific American

If Spacetime Were a Superfluid, Would It Unify Physics—or Is the Theory All Wet? - Scientific American: If it is true that spacetime is a superfluid and that photons of different energies travel at different speeds or dissipate over time, that means relativity does not hold in all situations. One of the main tenets of relativity, the Lorentz invariance, states that the speed of light is unchanging, regardless of an observer’s frame of reference. “The possibility that spacetime as we know it emerges from something that violates relativity is a fairly radical one,” Jacobson says. It does, however, clear a potential pathway toward rectifying some of the problems that arise when trying to combine relativity and quantum mechanics. “Violating relativity would open up the possibility of eliminating infinite quantities that arise in present theory and which seem to some unlikely to be physically correct.”

Stephen Hawking: 'There are no black holes' : Nature News & Comment

Stephen Hawking: 'There are no black holes' : Nature News & Comment: Now Hawking proposes a third, tantalizingly simple, option. Quantum mechanics and general relativity remain intact, but black holes simply do not have an event horizon to catch fire. The key to his claim is that quantum effects around the black hole cause space-time to fluctuate too wildly for a sharp boundary surface to exist.

In place of the event horizon, Hawking invokes an “apparent horizon”, a surface along which light rays attempting to rush away from the black hole’s core will be suspended. In general relativity, for an unchanging black hole, these two horizons are identical, because light trying to escape from inside a black hole can reach only as far as the event horizon and will be held there, as though stuck on a treadmill. However, the two horizons can, in principle, be distinguished. If more matter gets swallowed by the black hole, its event horizon will swell and grow larger than the apparent horizon.

Curved Spacetime Mimicked on a Chip: Scientific American

Curved Spacetime Mimicked on a Chip: Scientific American: Liu and his collaborators simulated the gravitational lensing of a star on an integrated photonic chip. A layer of clear plastic on the chip acted as a waveguide, confining light to the chip’s surface. To change the index of refraction of the plastic, the researchers had to vary the plastic’s thickness. They did so by heating the plastic and adding polystyrene microspheres before the plastic cooled. Because the plastic rose upwards around the microspheres on cooling, the thickness of the waveguide increased near these miniature balls. The varying index of refraction the team achieved happens to be very similar to the bending of space-time geometry around a massive star...

“This is indeed the first time an exact solution of Einstein's equations was mimicked” using an optical model, says Leonhardt.

'Black holes' of the ocean could curb climate change - environment - 26 September 2013 - New Scientist

'Black holes' of the ocean could curb climate change - environment - 26 September 2013 - New Scientist: Swirling masses of water in the ocean are mathematically the same as the warped regions of space-time around cosmic singularities...

In this so-called photon sphere, light is trapped in loops that spin around the black hole forever.


"The boundaries of water-carrying eddies satisfy the same type of differential equations that the area surrounding black holes do in general relativity..."

Curves in spacetime violate Heisenberg's uncertainty principle

Curves in spacetime violate Heisenberg's uncertainty principle: "Deutsch's model describes the strange quantum effects that we might see in the presence of CTCs, within a future theory of quantum gravity," Pienaar said. "However, if there are no CTCs in the universe, then we would not expect to see the effects. But since the slowing of time due to gravity looks just like the effect of an OTC from the outside, and since OTCs still lead to strange effects (as we have shown), we suggested that these effects might turn up in strong gravitational fields, even without any closed loops in time. If so, then they would allow us to violate the Heisenberg uncertainty principle and clone coherent states of light without needing a full-blown time machine.

"Of course, the connection between OTCs and gravitational fields is still very speculative and might turn out to be wrong..."

Physicists Find a Backdoor Way to Do Experiments on Exotic Gravitational Physics

Physicists Find a Backdoor Way to Do Experiments on Exotic Gravitational Physics: But what about running the dualities in the other direction, using laboratory measurements of extreme materials to probe exotic gravitational physics?...
The experiments in question entail smashing gold or lead nuclei together to create plasmas of quarks and gluons...
The plasmas must actually be liquid...

They equated the viscosity of a fluid to gravitational waves caroming off a black hole in higher-dimensional space—which, even for a physicist, is not an analogy that springs to mind...

The answer: 1/4π, in the appropriate units. The viscosity measured by RHIC comes close. Water, some 400 times more viscous, is molasses in comparison.

Surprisingly, the minimum value is the same for all fluids, whatever they are made of. Through the logic of duality, this universality has a simple explanation: Viscosity is equivalent to a gravitational phenomenon, and according to Einstein’s general theory of relativity, gravitation is blind to compositional details.

The real reason why the Pioneer spacecrafts appear to be slowing down

The real reason why the Pioneer spacecrafts appear to be slowing down: According to Sergei Kopeikin, the previous explanation for the so-called Pioneer anomaly was only able to account for 15 to 20% of the observed deceleration. Kopeikin, on the other hand, devised a new set of calculations which factored in the expansion of the universe — including the way it affects the movement of photons that make up light and radio waves.

'Astronomical Unit,' or Earth-Sun Distance, Gets an Overhaul

'Astronomical Unit,' or Earth-Sun Distance, Gets an Overhaul: The new standard... is now 149,597,870,700 meters...

Most recently, the au was defined as...: “the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day...

Pioneer Spacecraft Warmth Takes Heat off Relativity

Pioneer Spacecraft Warmth Takes Heat off Relativity: Pioneer 10 and 11 are slowing down due to a small-but-ever-present thermal recoil. Both spacecraft give off heat from their electronics and from the radioactive decay of their plutonium fuel, and that’s enough to impart the measured deceleration. The researchers liken it to photons from a car’s headlights pushing gently back on the vehicle. The analysis is in the journal Physical Review Letters.

Astrophile: The case of the disappearing pulsar - New Scientist - New Scientist

Astrophile: The case of the disappearing pulsar: The star was a spectacular find: unlike every other pulsar ever observed, this one was in a close binary orbit with another pulsar. Together, the pair provided a precise laboratory to test Einstein's theory of general relativity, and a means of detailing how pulsars behave.
But in March 2008, Pulsar B went dark...
No one snuffed out Pulsar B – it just rotated out of view...

"We can see the light from one pulsar being bent as it travels through the gravitational well of the other pulsar," she says. "It's really neat. We have proof that one of these objects is distorting spacetime."
The eclipsing pulsars also provided a test of "spin precession", the idea that the pulsars' axes should wobble around like a top as they spin.

BBC News - Survey gets a grip on dark energy

BBC News - Survey gets a grip on dark energy: "What we've done is measure the 3D positions for just over 250,000 galaxies, covering the largest volume of the Universe ever surveyed. That gives us an amazing map that we can then analyse to try to understand how and why the Universe's expansion is accelerating..."


One concerns so-called baryon acoustic oscillations. These are pressure-driven waves that passed through the very early Universe and which were imprinted on the distribution of matter once conditions had cooled below a certain point.

Today, these "wiggles" show themselves as a preferred scale in the separation of galaxies and can be used as a kind of standard ruler to measure the geometry of the cosmos...

The other technique being used by BOSS involves "redshift space distortions". These describe the component in the velocity of galaxies that stems from the growth of structure in the Universe. The team can see if neighbouring galaxies are clustering in the way that would be expected from the action of gravity.

What a trip through a wormhole would look like

What a trip through a wormhole would look like: As you emerge from the black hole, you enter a wormhole where the flow of space turns around and you start to accelerate back outward. The wormhole ends at the entrance to a white hole, which is a time-reversed version of a black hole.  Instead of falling inward, space falls outwards at a speed faster than light.  Soon you experience another flash of radiation, this time containing a picture of the entire future of the universe.

[1203.0102] GPS test of the local position invariance of Planck's constant

[1203.0102] GPS test of the local position invariance of Planck's constant: Publicly available clock correction data from the Global Positioning System was analyzed and used in combination with the results of terrestrial clock comparison experiments to confirm the local position invariance (LPI) of Planck's constant within the context of general relativity. The results indicate that h is invariant within a limit of |beta_h|<0.007, where beta_h is a dimensionless parameter that represents the extent of LPI violation.

Largest dark matter map holds clues to dark energy

Largest dark matter map holds clues to dark energy: Heymans's team used the Canada-France-Hawaii Telescope to observe 10 million galaxies, each about 6 billion light years away. The researchers analysed the images to build a map of dark matter spanning 10 billion light years, the first direct glimpse of dark matter on such a large scale.
The map shows a great cosmic web, with scattered clumps of dark matter linked by wispy filaments...
The map also reaffirms the need for dark energy to account for the accelerating expansion of the universe. Some proposed alternatives say that, instead of invoking a whole new entity, physicists might simply need to tweak Einstein's theory of general relativity on very large scales. But the new map, which spans very large scales yet is consistent with general relativity, lets the air out of those theories. "So far," says Miller, "there is no observational evidence for any departure from Einstein's theory."

One clock with two times: When quantum mechanics meets general relativity

One clock with two times: When quantum mechanics meets general relativity: The team at the University of Vienna considers a single clock (any particle with evolving internal degrees of freedom such as spin) which is brought in a superposition of two locations – one closer and one further away from the surface of the Earth. According to general relativity, the clock ticks at different rates in the two locations, in the same way as the two twins would age differently. But since the time measured by the clock reveals the information on where the clock was located, the interference and the wave-nature of the clock is lost. "It is the twin paradox for a quantum 'only child', and it requires general relativity as well as quantum mechanics. Such an interplay between the two theories has never been probed in experiments yet..."