Why do some want to modify general relativity?

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And yes, I did refer to MOND as “a fungus in the basement of astronomy”.

Do We Need to Rewrite General Relativity?

For NOVA “The Nature of Reality”:

General relativity, the theory of gravity Albert Einstein published 100 years ago, is one of the most successful theories we have. It has passed every experimental test; every observation from astronomy is consistent with its predictions. Physicists and astronomers have used the theory to understand the behavior of binary pulsars, predict the black holes we now know pepper every galaxy, and obtain deep insights into the structure of the entire universe.

Yet most researchers think general relativity is wrong.

To be more precise: most believe it is incomplete. After all, the other forces of nature are governed by quantum physics; gravity alone has stubbornly resisted a quantum description. Meanwhile, a small but vocal group of researchers thinks that phenomena such as dark matter are actually failures of general relativity, requiring us to look at alternative ideas. [Read the rest at NOVA…]

Calvin has it right.

“Dark energy” is one of the more unfortunate names in science. You’d think it has something to do with dark matter (itself a misnomer), but it has the opposite effect: while dark matter drives the clumping-up of material that makes galaxies, dark energy pushes the expansion of the Universe to greater and greater rates. Though we should hate on the term “dark energy”, we should respect Michael Turner, the excellent cosmologist who coined the phrase. He is also my academic “grand-advisor”: he supervised Arthur Kosowsky’s PhD, and Arthur in turn supervised mine.

And of course, I worked on dark energy as a major part of my PhD research. In my latest piece for Slate, I describe a bit of my dysfunctional relationship with cosmic acceleration, and why after 16 years dark energy is still a matter of frustration for many of us.

Because dark energy doesn’t correspond easily to anything in the standard toolkit of physics, researchers have been free to be creative. The result is a wealth of ideas, some that are potentially interesting and others that are frankly nuts. Some string theorists propose that our observable universe is the result of a vast set of parallel universes, each with a different, random amount of dark energy. Other physicists think our cosmos is interacting with a parallel universe, and the force between the two drives cosmic acceleration. Still others suspect that dark energy is a sign that our currently accepted theory of gravity—Einstein’s general theory of relativity—is incomplete for the largest distances. [Read more…]

My dysfunctional relationship with dark energy

The week in review (October 20-26)

Evidently, Nicole "the Noisy Astronomer" Gugliucci did not like it when I quoted Star Wars at her. All I said was "Aren't you a little short for a Stormtrooper?" [Credit: Melanie Mallon]

Evidently, Nicole “the Noisy Astronomer” Gugliucci did not like it when I quoted Star Wars at her. All I said was “Aren’t you a little short for a Stormtrooper?” [Credit: Melanie Mallon]

I had a wonderful time at GeekGirlCon; thanks again to Dr. Rubidium, AKA Nick Fury, for putting together the DIY Science Zone, and to everyone who made it a great event. I have a more formal wrap-up post in the works, but in the meantime, have some science writing.

  • The river of spacetime (Galileo’s Pendulum): As a follow-up to my earlier post, I extended the metaphor of dynamic spacetime. If spacetime is the river, gravity is the current, carrying matter and light along with it.
  • New type of quantum excitation behaves like a solitary particle (Ars Technica): In materials, the relevant entities aren’t particles, but quasiparticlesThese are quantum excitations that have mass, charge, spin, and all that jazz, but those properties depend on the specifics of the material…and of external influences. So, physicists would like to create quasiparticles that are less finicky, and behave more like free, solitary particles. That type of excitation is a leviton, and experimenters created them for the first time, as described in this new paper.
  • Taking Measure: A ‘New’ Most Distant Galaxy (Universe Today): It seems that every week, we see a new “most distant galaxy” announcement. However, this new find is special for two reasons: it’s a rare case where astronomers have measured the distance accurately using the galaxy’s spectrum, and the specific galaxy is producing new stars at a much higher rate than expected. Also, this is my first contribution to Universe Today!
  • For the love of Gauss, please stop (Galileo’s Pendulum): A somewhat ranting post in which I get grumpfy about the over-use and misuse of certain examples from the history of science in popular science writing.
  • What do we call a theory that is no longer viable? (Galileo’s Pendulum): As a follow-up to that previous post, I ponder better ways to think about the history of science, and propose (somewhat seriously) a term to describe theories that were once viable, but are now ruled out by evidence.

The week in review (October 13 – 19)

I’m at GeekGirlCon this weekend, so I’m busy with non-writing activities as part of the DIY Science Zone. Thanks to our Fearless Leader Dr. “Nick Fury” Rubidium for putting our part of the event together!

  • Where Nature Hides the Darkest Mystery of All (Nautilus): Even though there’s no solid barrier, the event horizon of a black hole provides a boundary through which we can’t see or probe. That leads to a troubling idea: will we ever know what’s really inside that event horizon? Is there any way to learn about the interior by indirect measurements?
  • Black hole hair and the dark energy problem (Galileo’s Pendulum): Building off that article, what happens if our standard theory of gravity is modified? That’s not an entirely crazy idea: several modifications to general relativity have been proposed, inspired by inflation (the rapid expansion during the cosmos’ earliest moments) or dark energy. A recent paper examined that idea, and here’s my take.
  • Strongly magnetic pulsar could explain anomalous supernovas (Ars Technica): Some supernovas are particularly bright, especially some from the early Universe. These, known as “pair-instability” supernovas, are the explosion of very massive stars made of nearly pure hydrogen and helium. However, some of these super-luminous supernovas don’t quite fit that profile, including being too close. A new set of observations may show they are actually driven by a magnetar, a highly magnetized pulsar.
  • Gravitational waves show deficit in black hole collisions (Ars Technica): Mergers of supermassive black holes should happen frequently enough to produce a bath of gravitational radiation permeating the cosmos. While that gravitational wave background (GWB) possesses wavelengths too large for ground-based detectors like LIGO, astronomers realized it might be visible in the fluctuations of light from pulsars. However, they didn’t see what they expected, leading to the big question: why not?

Two weeks in review (September 29 – October 12, 2013)

The center of the Milky Way lies at the upper left of this image from the 2MASS survey of galaxies. [Credit: 2MASS/G. Kopan, R. Hurt]

My black holes class and other responsibilities ate my brain the last two weeks, so I forgot to post a “week in review” last week. So, here’s the highlights from the last two weeks. If it’s more heavily weighted toward black holes even than usual, that’s hardly surprising.

  • Of fire and ice and Harlow Shapley (Galileo’s Pendulum): In 1918, a poet named Robert Frost met an astronomer named Harlow Shapley. The result, according to Shapley, was “Fire and Ice”. Most people probably don’t remember who Shapley was anymore, but in his day he was one of the most prominent astronomers, helping to map the galaxy and measuring its size.
  • Portrait of a black hole, part 1 (Galileo’s Pendulum): When trying to understand the curved four-dimensional spacetime of gravity, we have to resort to metaphor and simplified pictures. Here’s my attempt to describe spacetime around a (non-rotating) black hole using a dynamic analogy: a flowing current, against which objects must move.
  • A scientific love affair (Galileo’s Pendulum): Like many (most?) little kids, dinosaurs captured my imagination, sparking me to think about science for the first time. However, black holes, pulsars, and other products of extreme gravity inspired me in a different direction when I was in sixth grade. Here’s a partial story of my love affair with gravity.
  • The 2013 Nobel Prize in physics: the Higgs boson (Galileo’s Pendulum): The 2013 Nobel Prize was awarded this week to François Englert and Peter Higgs for the theoretical prediction of what is now known as the Higgs boson. This post celebrates that award, but also delves into how the Nobel Prize fails. In promoting the “lone (male) genius” view of science and thereby failing to acknowledge contributions by the others who deserve recognition for the Higgs boson, the Nobel Prize does a disservice to that which it seeks to honor. Bonus: what the Nobel Prize has to do with the leg lamp from A Christmas Story.
  • Measuring a superconducting qubit by manipulating its environment (Ars Technica): Now for something completely different! Quantum systems are complicated, involving interactions between the objects we want to study, the environment of those objects, and our measuring apparatus. A new experiment shows a way of measuring an object’s properties indirectly by performing environmental measurements instead. The result is a picture of a superconducting quantum bit (or qubit) as it evolves in time.

The week in review (August 18-24)

Granulation on the surface of the Sun, created by rising bubbles of hot plasma. Fluctuations in these bubbles can be measured on distant stars, which provides a way to calculate the stars' surface gravity. [Credit: Hinode JAXA/NASA/PPARC]

Granulation on the surface of the Sun, created by rising bubbles of hot plasma. Fluctuations in these bubbles can be measured on distant stars, which provides a way to calculate the stars’ surface gravity. [Credit: Hinode JAXA/NASA/PPARC]

I’ve been remiss in blogging at Bowler Hat Science, largely because…well, I’ve been writing too much elsewhere. So, I’m going to try something different: instead of blogging each new article I write in a separate entry, I’ll write a single post summarizing everything in one go.

  • How I learned to stop worrying and love tolerate the multiverse (Galileo’s Pendulum): My explanation of cosmology involving parallel universes is a response to a piece placing the multiverse in the same category as telepathy. While I’m not a fan of the multiverse concept, I reluctantly accept that it could be a correct description of reality.
  • An Arguably Unreal Particle Powers All of Your Electronics (Nautilus): Electrons in solids behave differently than their wild cousins. In some materials, the electronic and magnetic properties act as though they arise from particles that are lighter or heavier than electrons, or multiple types of particles with strange spins or electric charges. Are these quasiparticles real?
  • Kepler finds stars’ flickers reveal the gravity at their surface (Ars Technica): The Kepler observatory’s primary mission was to hunt for exoplanets, but arguably it’s been equally valuable for studying stars. A new study revealed a way to measure a star’s surface gravity by timing short-duration fluctuations — the rippling of hot plasma bubbles on the surface known as granulation (see above image).
  • Destruction and beauty in a distant galaxy (Galileo’s Pendulum): The giant galaxy M87 has a correspondingly huge black hole at its heart. That black hole in turn generates an enormous jet of matter extending 5,000 light-years, which fluctuates in a way we can see with telescopes. In that way, an engine of destruction shapes its environment and produces a thing of beauty.
  • The Freaky Celestial Events We See—and the Ones We Don’t (Nautilus): In another faraway galaxy, a black hole destroyed a star, producing a burst of gamma rays that lingered for months. This event is the only one of its kind we’ve yet seen, prompting the question: how do we evaluate events that are unique? How can we estimate how likely they truly are, especially if we’re seeing them from a privileged angle?
  • This isn’t writing, but after listing two black hole articles in a row, it seems a good time to advertise my Introduction to Black Holes online class in October! Sign up to learn all* about black holes. *All = what I can cover in four hours of class time.
  • Warp Speed? Not So Fast (Slate): Many articles have appeared over the last year or so profiling a NASA researcher, whose research supposedly could lead to a faster-than-light propulsion system. The problem: very little actual information about his work is known, and what he’s said publicly contradicts what we understand about general relativity and quantum physics.

Speaking of warp drives, I’ll conclude with this wonderful video of Patrick Stewart engaging with some obvious Star Trek fans.

General relativity holds up under extreme gravity test

The general theory of relativity is the reigning champion of gravitational theories: it’s withstood tests in the Solar System, near black holes, and in binary systems. Most recently, astronomers performed detailed observations of a pulsar-white dwarf binary system, which provided an exquisite example of general relativity in action. Pulsars and white dwarfs are both the remnants of stars, but pulsars in particular are interesting: they pack the mass of a star into a sphere about 20 kilometers across. That means the gravity at the surface of a pulsar is extreme, so when one is in a binary system, it provides a laboratory for measuring strong gravitational effects.

The pulsar itself was interesting because of its relatively high mass: about 2.0 times that of the Sun (most observed pulsars are about 1.4 times more massive). Unlike more mundane objects, pulsar size doesn’t grow with mass; according to some models, a higher mass pulsar may actually be smaller than one with lower mass. As a result, the gravity at the surface of PSR J0348+0432 is far more intense than at a lower-mass counterpart, providing a laboratory for testing general relativity (GR). The gravitational intensity near PSR J0348+0432 is about twice that of other pulsars in binary systems, creating a more extreme environment than previously measured. [Read more…]

Also, let the record show: it’s possible to write an article about testing general relativity without mentioning Einstein, much less making the story about “proving him right” (or wrong).

The region near a black hole is one of the most extreme environments in the Universe, but historically it’s been hard to study directly. Using the XMM-Newton and NuSTAR telescopes, astronomers have measured the rotation of gas near the supermassive black hole at the center of the Great Barred Spiral Galaxy. They found that this black hole is spinning nearly as fast as it can be, and that the matter orbiting the black hole is similarly moving near the speed of light—to the extent that the results of Einstein’s general relativity must be used to understand how it’s moving. The key measurement involved observing X-rays reflected off the matter whirling around the black hole, a significant observation of a relativistic phenomenon.

A new X-ray observation of the region surrounding the supermassive black hole in the Great Barred Spiral Galaxy may have answered one of the big questions. G. Risaliti and colleagues found the distinct signature of X-rays reflecting off gas orbiting the black hole at nearly the speed of light. The detailed information the astronomers gleaned allowed them to rule out some explanations for the bright X-ray emission, bringing us closer to an understanding of the extreme environment near these gravitational engines. [Read more…]

Measuring the spin of a black hole using X-rays