The physics of the universe
Switch off fusion and, for ten thousand years, nothing happens. Then the Sun begins a slow, strange death: shrinking, briefly brightening, and coasting on gravitational heat for tens of millions of years. And the neutrinos give the whole thing away in just eight minutes.
A photon born in the Sun's core takes around 100,000 years to fight its way to the surface, bouncing through a random walk so inefficient that the light on your face is older than human civilization. Why the Sun's surface is a hundred-millennia-delayed broadcast.
How can the Sun keep shining with its furnace switched off? Two nineteenth-century aristocrats, Helmholtz and Lord Kelvin, worked out the answer mostly by accident. It comes down to stored heat, gravitational shrinking, and the strange self-regulating thermostat of hydrostatic equilibrium.
If the Sun's fusion shut off right now, you would not notice for a very long time. The first stop is understanding the Sun itself: a vast pile of gravitating matter where fusion is so absurdly inefficient that, pound for pound, a compost heap beats it.
The ekpyrotic universe is a beautiful idea that runs headlong into the data. From hand-waved singularities and assumed dark energy to the killer blow from Planck and WMAP measurements of the cosmic microwave background, here is why nature has so far voted against it.
The ekpyrotic theory tries to beat inflation with bouncing higher-dimensional branes, no singularity, and a universe that has always existed. A tour of the prettiest version of the idea and how it claims to handle flatness, dark energy, and the entropy that doomed earlier cyclic models.
An international team led by astronomers at the University of Sydney has uncovered the clearest evidence yet for the origin of an unusual class of cosmic signals. In doing so, they have identified a rare stellar system that is providing scientists with a natural laboratory to study extreme physics.
Inflation is awkward, possibly not even a proper theory, and it has reigned over cosmology for forty years anyway. Here is what it claims, the flatness, horizon, and monopole problems it solves, the structure-formation prediction it nailed, and the deep problems it still cannot escape.
A look at why a cyclic, eternally repeating universe is such an appealing idea, and why the first serious attempt to build one, Richard Tolman's 1930s model of endless big bangs and big crunches, collapsed under the weight of entropy. The Big Bang keeps demanding a beginning.
The LIGO–Virgo–KAGRA (LVK) detector network has a new trick up its sleeve to improve the instruments’ sensitivity to gravitational waves: it’s called Astrophysical Calibration and it plays a role similar to auto-tune in music production.
New research led by Penn State scientists suggests that some of the highest-energy cosmic rays may consist of atomic nuclei heavier than iron and could help narrow down the cosmic sources capable of accelerating these particles.
No dust, no way to cool a collapsing gas cloud. No way to cool it, no stars. No dust, no first rung on the ladder from grain to pebble to planet. The substance I spent two articles complaining about turns out to be the substance that makes me possible.
Two hydrogen atoms can't form an H2 molecule on their own in empty space. They need a surface. The universe has only one surface available, and it's something I have just spent two articles complaining about.
Dust scatters light, absorbs light, re-emits light, and ruins everything. It's why our maps of the Milky Way were wrong before 1930, and it's why one of the biggest cosmological announcements of the 2010s quietly evaporated.
Neutrinos, the second most common fundamental particles in the universe, are notoriously difficult to detect. So far we’ve only been able to do so by building giant vats of water far underground with hundreds of photodetectors watching for brief flashes of light. But a new CubeSat mission hopes to change that dynamic and enable the neutrino detectors of the future a much less constrained and expensive existence - in space.
Years of grievance against dust. It ruins lungs, suits, rovers, and Mars missions. The first installment of an apology, sort of, to the most annoying substance in the cosmos.
No quantum gravity. The wrong peak in the wave function. Boltzmann Babies. Roger Penrose pointing out that the arrow of time was smuggled in through the back door. The no-boundary proposal is beautiful. It is also possibly wrong in many specific ways.
Run Hawking's machinery and out pops something startling: the most likely universe looks an awful lot like ours, complete with inflation, a low-entropy beginning, and an arrow of time. All of cosmology, falling out for free. Almost.
Hawking faced a question with no answer hiding behind it. The best boundary condition for the universe, he decided, was that there was no boundary at all. To make that statement into physics, he had to do something deeply strange to time.
The equations of general relativity give up at the singularity. Decades before Stephen Hawking dared to guess what came before, John Wheeler and Bryce DeWitt built the strange mathematical machinery that would make the question askable in the first place.
Universe Today