Why do some want to modify general relativity?

[ This blog is dedicated to tracking my most recent publications. Subscribe to the feed to keep up with all the science stories I write! ]

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…]

No quantum foam seen in the cosmic beer glass

[ This blog is dedicated to tracking my most recent publications. Subscribe to the feed to keep up with all the science stories I write! ]

Light from distant black holes doesn’t surf on waves of quantum foam

Strongest check yet on quantum gravity effects in astronomy turns up nothing

For Ars Technica:

Quantum gravity is notoriously slippery. While the Standard Model successfully describes three forces of nature, it doesn’t include gravity, so gravity still has no consistent quantum theory. To make matters worse, gravity is so weak that it’s difficult to probe at the sorts of energies where any minuscule quantum effects would pop out. However, some researchers predict that those tiny effects could accumulate over cosmological distances: light traveling from far-off quasars would be changed by the “quantum foam” of spacetime, producing blurry images in our telescopes—or even making objects seem to disappear.

A new report by E. S. Perlman and colleagues examines the disappearance hypothesis using gamma-ray data from quasars. In particular, they investigated a possibility suggested by the holographic principle, the idea that all the information in the cosmos can be encoded on the two-dimensional boundary that encloses it. Disappointingly for fans of quantum foam, the gamma ray data did not show any measurable fading or blurring of the quasars.

As the authors point out, these results don’t rule out anything general regarding quantum gravity, quantum foam, or the holographic principle. But they do provide the tightest constraint yet on cumulative effects of quantum foam on light traveling across the Universe. [Read the rest at Ars Technica…]

This metal plate is perforated with holes, each of which lines up with a galaxy or quasar. The BOSS survey maps the position and distance to a huge number of galaxies using many masks such as this. [Credit: moi]

This metal plate is perforated with holes, each of which lines up with a galaxy or quasar. The BOSS survey maps the position and distance to a huge number of galaxies using many masks such as this. [Credit: moi]

Far from being invisible, black holes are among some of the brightest objects in the Universe. The black holes themselves aren’t emitting light, but the matter they draw in heats up and much of it shoots back out in powerful jets. When that happens, the black hole is known as a quasar, and it can be visible from billions of light-years away. For that reason, mapping the distribution of quasars can help cosmologists understand the expansion rate of the Universe in an earlier era — and constrain the behavior of dark energy. My latest story in The Daily Beast explains:

If dark energy will be the same in billions of years as it seems to be today, the future will be dark and empty, as galaxies continue to move apart from each other at ever-faster rates. If dark energy comes and goes, though, maybe the rate of expansion will slow down again. All of this is a long time from now—trillions of years after the death of the Sun—but we might see hints about it today. We hope to see signs of what is to come by looking at how dark energy behaves now, and how it has acted in the past. Similarly, if dark energy is stronger in some parts of the cosmos, then certain pockets of the Universe would grow faster than in others. That also has implications for how the future cosmos looks. [Read more…]

Using Black Holes to Measure Dark Energy, Like a BOSS

Astronomers measured the rotation of a black hole from halfway across the Universe.

What, I need to say more?

Astronomers have now used gravitational magnification to measure the rotation rate of a supermassive black hole in a very distant galaxy. From four separate images of the same black hole, R.C. Reis, M.T. Reynolds, J.M. Miller, and D.J. Walton found it was spinning nearly as fast as possible. That likely means it was spun up by a small number of mergers with other black holes rather than a gradual increase from eating smaller amounts of mass.

This marks the first measurement of black hole rotation outside the local Universe…. [Read more]

Measuring black hole rotation halfway across the Universe

How did the biggest black holes form?

X-ray image of two black holes in the galaxy NGC 6240. Binary systems like this are possibly the origin of the most massive black holes in the cosmos. [Credit: NASA/CXC/MPE/S.Komossa et al. ]

X-ray image of two black holes in the galaxy NGC 6240. Binary systems like this are possibly the origin of the most massive black holes in the cosmos. [Credit: NASA/CXC/MPE/S.Komossa et al.]

The most massive known object in the cosmos is the black hole at the center of M87, a huge galaxy in the Virgo Cluster. While most large galaxies (including the Milky Way) harbor supermassive black holes, the very largest are interesting. That’s because galaxies and their black holes seem to share a history, based on the relationship between the mass of the black hole and the mass of the galaxy’s central region. Since large galaxies grew by devouring smaller galaxies, or by two galaxies merging into a larger one, it’s very likely the biggest black holes followed a similar process. My latest piece for Nautilus examines how this process might have taken place, and what it could reveal about the black holes themselves.

Earth emits gravitational waves as it orbits the Sun, though the amount of energy lost is imperceptible over the lifetime of the Solar System. Binary black holes are a different matter: Once they are relatively close, they shed a tremendous amount of energy, bringing them closer together with each orbit. (Binary black stars are thought to emit more gravitational energy as they merge than regular stars emit in the form of UV, IR, and visible light over their entire lifetimes of billions of years.) Eventually their event horizons will touch, and the system emits a lot more gravitational waves in a phase known as “ring-down,” as the lumpy, uneven merged mass becomes a smooth, perfectly symmetrical black hole. [Read more…]

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?

The Solar System boundary and the week in review (September 8-14)

Cthulhu at NASA Wallops, for the LADEE launch last weekend. (I didn't wear the hat the whole time. I'm not that weird.)

Cthulhu at NASA Wallops, for the LADEE launch last weekend. (I didn’t wear the hat the whole time. I’m not that weird.)

‘Twas a busy week!

  • High-resolution observations show how black hole jets churn galactic gas (Ars Technica): One portion of my PhD thesis involved galactic feedback. That’s the process by which jets from black holes at the center of galaxies push material away, potentially affecting star formation and other activity. This article addressed the observation of galactic feedback, showing exactly where the hot jet of plasma from the black hole meets the colder atoms in galactic clouds. Very awesome stuff!
  • Parallel Earth and the Evil Matthew Hypothesis (Double X Science): I don’t know if Star Trek was the original source of the “evil twin from a parallel world” trope, but it’s the most famous. The idea is that there’s a mirror universe to ours, in which things are almost the same, but not quite. I discussed that trope in light of the multiverse, the concept that during rapid expansion right after the Big Bang, the Universe split into a number of disconnected regions that might obey different laws than our own.
  • Do-it-yourself science at GeekGirlCon (Galileo’s Pendulum): We’re still raising money to send a group of us to GeekGirlCon in Seattle next month! We’re willing to embarrass ourselves in public to accomplish this! However, the real purpose is to have hands-on science activities at the con.
  • Status of the book-in-progress (Galileo’s Pendulum): On a more somber note, I have suspended work on my book indefinitely and released my agent. I haven’t completely given up on either the book or getting it published, but the frustrations around the whole process have exhausted me, so it’s time for a break.
  • Cosmic coincidence and a potato eclipse (Double X Science): The Moon is nearly the same size as the Sun in our sky, which has led to all sorts of mystical musings and apocalyptic fears, especially during eclipses. However, that appearance is a coincidence, which we can understand using simple geometry. What’s even more fun to contrast our Moon to Phobos, the larger of Mars’ two moons, which is much smaller than our own but manages to create its own eclipses.
  • Voyager 1 really has left the Solar System…probably (Ars Technica): Sometime last year, the venerable spacecraft Voyager 1 crossed into interstellar space. While there have been a lot of announcements along these lines (I compared the number with Spinal Tap drummers), this time the probe seems to have actually done it. The necessary measurement is the plasma density, which is much higher in interstellar space, but Voyager’s plasma instrument had been knocked out by a solar flare. Researchers pieced together the appropriate data from other instruments. There’s still an anomalous measurement that needs to be accounted for — the magnetic field doesn’t behave as predicted — but I think it’s pretty safe to say that’s an issue for theorists, not ambiguity about Voyager’s position. (See below for a discussion of whether Voyager has actually “left the Solar System” or not.)
  • Mapping the dark matter in the tiniest of galaxies (Galileo’s Pendulum): Dwarf spheroidal galaxies don’t look like galaxies at all. They have so few stars and so little gas or dust, they’re nearly see-through, yet they have as much as 1000 times more dark matter than ordinary matter. (In regular galaxies, dark matter is more like 10 times the amount.) Two astronomers analyzed the motion of stars within dwarf spheroidals to see if they could map the distribution of dark matter, and they found something similar to what is seen in larger galaxies.
  • Finally, I participated in the Weekly Space Hangout, sponsored by Universe Today and CosmoQuest. I joined hosts Fraser Cain and Nicole Gugliucci, along with Amy Shira Teitel, David Dickinson, and Nancy Atkinson to talk about the space and astronomy news from the last week. The whole thing is archived at Google+, or you can watch the video on YouTube.

Where’s the edge of the Solar System?

Returning to Voyager 1, I think stories about its passage into interstellar space fell into two major categories: those saying “Voyager 1 has left the Solar System!” and variations on “Stop saying Voyager 1 has left the Solar System!” Despite what the headline on my story said, the second group of people (which includes writers I respect like Phil Plait and Amy Shira Teitel) is correct: the Solar System includes the Oort Cloud, a diffuse region of icy bodies loosely bound to the Sun by gravity.

A radio image of Voyager 1, as seen by the Very Long Baseline Array (VLBA) and the Green Bank Telescope. Click for a larger image and more information. [Credit: Alexandra Angelich, NRAO/AUI/NSF]

A radio image of Voyager 1, as seen by the Very Long Baseline Array (VLBA) and the Green Bank Telescope. Click for a larger image and more information. [Credit: Alexandra Angelich, NRAO/AUI/NSF]

However, if you want to say Voyager has left the Solar System, I’ll back you up: the boundary between the Oort Cloud and the “rest of the galaxy” isn’t very well defined. Gravity technically extends forever, though it weakens substantially with larger distances. As a result, the Oort Cloud is a fuzzy edge, and one we can’t measure. Is the end of the Solar System the point where the last Oort Cloud body resides?

Now, I agree with the pedants that the Oort Cloud truly does define the end of the Sun’s influence, and therefore is the edge of the Solar System. But the magnetic boundary of the Solar System, which is arguably the more important one from the point of view of astronomy, is defined by the edge of the heliopause, where the solar wind hits interstellar gas. That boundary, while it fluctuates with solar weather, is a much clearer division, and one we could conceivably measure near other stars.

So, I’m a both/and kind of guy in this case. Since there’s no single, sharp boundary between the Solar System and “everything else”, let’s just say there are two edges: one for the Sun’s electromagnetic influence (the heliopause), and one for its gravitational influence (the Oort Cloud). Voyager crossed the first one, but won’t reach the second one for 300 years. Now, can we get back to talking about how awesome Voyager is?