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Pulsars spread across the galaxy are dancing to the same fiddler
Pulsars emit radio signals in pulses many times a second. Though these pulses are very regular, there is a small wobble in the timing of each. We learned this year that all 68 known fast pulsars are wobbling in unison, though they are thousands of lightyears apart. What gives?
If a star is big enough to explode at the end of its natural life, but not big enough to form a black hole, it will explode on the outside (from a burst of energy) while collapsing on the inside (from gravity).
Collapse may be halted when the core of the star reaches a kind of liquid state, where the atoms are pushed tightly together. This is called a “white dwarf” star. It is white because it is hotter than the hottest main sequence star, and it is a dwarf because it is only about the size of the earth (which is a million times smaller than the sun).
This is actually the same phenomenon that makes familiar, earthbound liquids and solids so resistant to compression. Electrons are Fermions, and Fermions avoid occupying the same quantum state. We think of liquids and solids as incompressible, but in the case of white dwarf stars, the pressure from gravity is so tremendous that the hydrogen-helium mixture is compressed to an unimaginable level, about a million times denser than familiar materials like water or stone.
Sometimes, the liquid state fails and the collapse continues. Then there is one more trap, beyond which collapse into a black hole is inevitable. Collapse may be halted when all the electrons and protons merge to form neutrons, and all the neutrons come together in one ginormous atomic nucleus. The neutrons resist further compression because, like electrons, neutrons are also Fermions. Because a neutron is two thousand times heavier than an electron, it maintains its resistance up to much higher energies and pressures. (Ultimately what causes the pressure of both electrons and neutrons to soften is that their speeds get too close to the speed of light.)
You may have heard that the nucleus at the center of an atom is much, much smaller than the atom itself. The density of a neutron star is comparable to the density of an atomic nucleus, which is 10^15 times denser than familiar earth-bound materials.
Neutron stars condense all the mass of a star into a sphere just a few miles across. Everything about a neutron star is unbelievably extreme. Temperatures hotter than the center of the sun. Magnetic fields a billion times stronger than we can create with the biggest superconducting magnets. They are so dense that a spoonful of neutrons weighs a billion tons, and that’s based on our familiar Earth gravity. Gravity on a neutron star is a trillion times stronger.
Neutron stars spin anywhere from once per second to hundreds of times per second. With each spin, their radio beacon scans past our direction, and we see a radio pulse. Hence neutron stars are also called pulsars. About 3,000 pulsars have been discovered since Jocelyn Bell found the first one in 1966. 68 of them spin super-fast, for reasons that are not well understood. They’re called “millisecond pulsars”.
The timing of their pulses is quite precise, but our clocks are even more precise. We can detect that most pulsars are slowing down just a bit because they’re losing energy into space from all those radio waves. What’s more, there are tiny fluctuations in the pulsar periods, little wobbles in which the pulsars get a tiny bit faster and a tiny bit slower.
What if I told you that all those wobbles are coordinated? There is a synchrony to the wobbles of 68 of the fastest pulsars, scattered across our galaxy in different directions.
Wait a minute, you say. Pulsars a few thousand light years to the north of us and a few thousand light years to the south of us, all wobbling at the same time? It takes their light thousands of years to reach us, and the transit time is different for each pulsar. They’re not wobbling at the same time from anyone’s perspective except ours. So this must be something about us, not something about the pulsars.
Just so, I respond. Astrophysicists ain’t so dumb. Their favored hypothesis is that all the clocks on earth are getting slightly faster and slower in a coordinated fashion every few years.
But what would cause that? A gravitational wave stretches time as well as space. This is the conventional explanation for the coordinated wobbles. But these coordinated changes are happening over the course of several years. There is no known process that could create gravitational waves that are so huge that their period is measured in years (or, equivalently, wavelength in lightyears).
My guess is that this is one of those pregnant mysteries that awaits a brilliant young physicist to propose a daring new theory that spans many anomalies to offer us a new view of the cosmos. The standard Big Bang model first went in for emergency surgery in 1998, and the Docs prescribed a massive dose of cold, dark matter (CDM) and an even more massive dose of dark energy. The model has been on life support ever since.
Last year, I wrote about evidence that the universe is not the same in all directions, which violates a basic premise of the standard Big Bang. In January, I wrote about evidence that the redshift — used to gauge how far away galaxies are — may be caused by something other than distance. Most recently, it has been observed that, on a cosmic scale, matter started being drawn together by gravity much too fast in the early universe, and the clumping has lagged more and more until the present time.
I think we’re overdue for a radical rewrite of the Big Bang. This is one of the most interesting challenges facing science today. Who can rise to this challenge? Even to understand the many things that the Big Bang gets right and the Big Bang gets wrong requires years of post-graduate training. Can anyone come through this schooling with all the requisite knowledge, but retaining the fresh, beginner’s mind that can imagine something radically different from the framework they have been taught?
PS — I apologize for simplifying the story, perhaps to the point of distortion. There is more detail in the excellent video below by Dr Becky Smethurst. I should note that
They didn’t just correlate timing of the wobble as viewed from earth. They also corrected for the distance from earth to each pulsar and from each pulsar to every other pulsar, to create and summarize a huge data set of correlations.
Some but not all of the results they found fit a theory based on galaxy mergers, in which supermassive black holes at the centers of two merging galaxies can rotate about each other with a period just a few years. This generates gravitational waves with a period of a few years.
To get these results, dozens of scientists collaborated and made adjustments and corrections to the data for more things than any one person can keep track of. This is a cause for skepticism of the whole endeavor. OTOH, four independent groups reported very similar conclusions.