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Even Albert Einstein himself didn’t believe we’d be able to detect gravitational waves from the merger of two black holes.

In late 2019 the previously unremarkable galaxy SDSS1335+0728 suddenly started shining brighter than ever before. To understand why, astronomers have used data from several space and ground-based observatories, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT), to track how the galaxy’s @brightness has varied. In a study out today, they conclude that they are witnessing changes never seen before in a galaxy – likely the result of the sudden awakening of the massive black hole at its core.

“Imagine you’ve been observing a distant galaxy for years, and it always seemed calm and inactive,” says Paula Sánchez Sáez, an astronomer at ESO in Germany and lead author of the study accepted for publication in Astronomy & Astrophysics. “Suddenly, its [core] starts showing dramatic changes in brightness, unlike any typical events we’ve seen before.” This is what happened to SDSS1335+0728, which is now classified as having an ‘active galactic nucleus’ (AGN) — a bright compact region powered by a massive black hole — after it brightened dramatically in December 2019 [1].

Some phenomena, like supernova explosions or tidal disruption events — when a star gets too close to a black hole and is torn apart — can make galaxies suddenly light up. But these brightness variations typically last only a few dozen or, at most, a few hundreds of days. SDSS1335+0728 is still growing brighter today, more than four years after it was first seen to ‘switch on’. Moreover, the variations detected in the galaxy, which is located 300 million light-years away in the constellation Virgo, are unlike any seen before, pointing astronomers towards a different explanation.

Katie Mack, Perimeter Institute’s Hawking Chair in Cosmology and Science Communication, takes us on journey through time by exploring the cosmic microwave background (CMB), the faint glow from the Big Bang.

The CMB shows us the universe as it was 13.8 billion years ago, revealing secrets about its…

Continue reading “Cosmic Microwave Background (CMB) Explained | Cosmology 101 Episode 2” »

Numerous open questions in gravity theory become apparent from observations in cosmology, such as the cosmic microwave background radiation [1, 2, 3, 4, 5, 6], the large scale structure [7, 8], gravitational waves [9, 10] and supernovae [11]. In order to describe these observations, one needs to study the evolution of both the universe as a whole, modeled by a homogeneous and isotropic background geometry and matter distribution, as well as perturbations of this background. A thorough understanding of such cosmological perturbations and their dynamics imposed by the gravitational interaction is therefore an important necessity for describing and explaining the modern observations in cosmology.

Cosmological perturbations in gravity have been studied for a long time, starting with the case of (pseudo-)Riemannian spacetime geometry, which is employed by the standard formulation of general relativity and the most well-known class of its extensions, in which the gravitational interaction is attributed to the curvature of the metric-compatible, torsion-free Levi-Civita connection [,13,14,15]. This task is significantly simplified by the fact by understanding how perturbations transform under gauge transformations, i.e., infinitesimal diffeomorphisms which retain the nature of the spacetime geometry as a small perturbation of a cosmologically symmetric background. From these gauge transformations, one can derive a set of gauge-invariant perturbation variables, which describe the physical information contained in the metric perturbations as well as the perturbations of the matter variables, so that they become independent of the arbitrary gauge choice. The resulting gauge-invariant perturbation theory is one of the cornerstones of modern cosmology [16,17,18,19].

Despite its overwhelming success in describing observations from laboratory scales up to galactic scales, general relativity is challenged by the aforementioned open questions, as well as the open question how it can be reconciled with quantum theory. This situation motivates the study of modified gravity theories [20]. While numerous theories depart from the standard formulation of general relativity in terms of the curvature of the Levi-Civita connection of a Riemannian spacetime, also other formulations in terms of the torsion or nonmetricity of a flat connection exist and can be used as potential starting points for the construction of modified gravity theories [21, 22]. Focusing on general relativity alone, one finds that these formulations are equivalent in the sense that they lead to field equations which possess the same solutions for the metric irrespective of the geometric properties of the connection under consideration…

Welcome to Rethinking the Foundations brought to you by Curt Jaimungal at Theories of Everything. Today, Professor Neil Turok answers questions from the audience! This is Part 2 to Neil’s Rethinking the Foundations episode. Part 1 can be seen here: • Rethinking the Foundations of Physics…

Listen on Spotify: https://open.spotify.com/show/4gL14b9…

Continue reading “Q&A About the Universe with Top Physicist Neil Turok” »

Have you ever placed the palm of your left hand on the back of your right hand in such a way that all fingers point in the same direction? If you have, then you probably know that your left thumb will not touch its right counterpart. Neither rotations nor translations nor their combinations can turn a left hand into a right hand and vice versa. This feature is called chirality.

But when it comes to the origin of the Universe, we don’t know what forces are at play. We actually can’t know, since to know such force (or better, such fields and their interactions) would necessitate knowledge of the initial state of the Universe. And how could we possibly glean information from such a state in some uncontroversial way? In more prosaic terms, it would mean that we could know what the Universe was like as it came into existence. This would require a god’s eye view of the initial state of the Universe, a kind of objective separation between us and the proto-Universe that is about to become the Universe we live in. It would mean we had a complete knowledge of all the physical forces in the Universe, a final theory of everything. But how could we ever know if what we call the theory of everything is a complete description of all that exists? We couldn’t, as this would assume we know all of physical reality, which is an impossibility. There could always be another force of nature, lurking in the shadows of our ignorance.

At the origin of the Universe, the very notion of cause and objectivity get entangled into a single unknowable, since we can’t possibly know the initial state of the Universe. We can, of course, construct models and test them against what we can measure of the Universe. But concordance is not a criterion for certainty. Different models may lead to the same concordance — the Universe we see — but we wouldn’t be able to distinguish between them since they come from an unknowable initial state. The first cause — the cause that must be uncaused and that unleashed all other causes — lies beyond the reach of scientific methodology as we know it. This doesn’t mean that we must invoke supernatural causes to fill the gap of our ignorance. A supernatural cause doesn’t explain in the way that scientific theories do; supernatural divine intervention is based on faith and not on data. It’s a personal choice, not a scientific one. It only helps those who believe.

Still, through a sequence of spectacular scientific discoveries, we have pieced together a cosmic history of exquisite detail and complexity. There are still many open gaps in our knowledge, and we shouldn’t expect otherwise. The next decades will see us making great progress in understanding many of the open cosmological questions of our time, such as the nature of dark matter and dark energy, and whether gravitational waves can tell us more about primordial inflation. But the problem of the first cause will remain open, as it doesn’t fit with the way we do science. This fact must, as Einstein wisely remarked, “fill a thinking person with a feeling of humility.” Not all questions need to be answered to be meaningful.

Researchers at Finland’s Aalto University have found a way to use magnets to line up bacteria as they swim. The approach offers more than just a way to nudge bacteria into order – it also provides a useful tool for a wide range of research, such as work on complex materials, phase transitions and condensed matter physics.

The findings have been reported in Communications Physics (“Magnetically controlled bacterial turbulence”).

Bacterial cells generally aren’t magnetic, so the magnets don’t directly interact with the bacteria. Instead, the bacteria are mixed into a liquid with millions of magnetic nanoparticles. This means the rod-shaped bacteria are effectively non-magnetic voids inside the magnetic fluid. When the magnets are switched on, creating a magnetic field, the bacteria are nudged to line up with the magnetic field because any other arrangement takes more energy – it’s harder to keep the rod-shaped holes at an angle to the magnetic field.

The findings could aid the hunt for these monstrous duos using gravitational waves, tiny ripples in space and time (united as a 4-dimensional entity called space-time), which were first predicted in Einstein’s theory of general relativity in 1915.

“These findings are useful for targeted searches for supermassive black hole binaries, in which we search specific galaxies and quasars for continuous gravitational waves from individual supermassive black hole binaries,” research lead author Andrew Casey-Clyde, a doctoral candidate at the University of Connecticut and visiting researcher at Yale University, told Space.com.

“Our results mean that these targeted searches will be up to seven times more likely to find gravitational waves from a supermassive black hole binary in a quasar than in a random massive galaxy,” Casey-Clyde said.