Could gravity have mass?

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Might gravity have mass?

Click on the image to read the whole article for free, courtesy of Physics World.

Click on the image to read the whole article for free, courtesy of Physics World.

From Physics World:

When confronted with something unexplained in the data, scientists face several possibilities. Maybe there’s an error and the result is spurious. Maybe there’s a more mundane explanation they simply overlooked. Or perhaps the unexplained is a sign that a theory needs to be revised or supplanted. That last option is the rarest, at least when the theory in ques- tion is a successful one. After all, any new theory must explain all the same phenomena an old theory explained, and predict something new that can’t be handled with the old.

One unexplained result that’s been bugging physicists for more than 15 years is dark energy, which is the name we give to our ignorance. The universe is expanding at an accelerating rate, but we don’t know why. To make matters worse, dark energy comprises roughly three-quarters of the total energy content of the cosmos, so it’s not a minor thing we don’t get. For that reason, a small but dogged group of physicists thinks the existence of dark energy might be a clue that we need to revise one of the most successful theories we have: general relativity.

One way to revise general relativity is to modify the nature of the gravitational force so that it behaves as though it has mass.

The rest of this story is in the print edition of Physics World, which you can subscribe to through membership in the Institute of Physics, which costs £15, €20, or $25 per year. You can join by clicking here. You can also get a nice mobile- and tablet-formatted version of the story using the Physics World app, available in the Google Play and iTunes stores. However, if you just want to read the rest of this article, Physics World has kindly allowed me to offer it to you as a PDF download, which looks exactly like the printed version!


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

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

I write articles and posts on a lot of different topics, both for my own blog and at Ars Technica. Many of those subjects drift pretty far from my putative area of expertise, but occasionally I get to write about something I know pretty well. To wit: last week, a group of researchers using the 4-meter Blanco telescope at Cerro Tololo (best known for its use in discovering dark energy in 1998) have measured distances to galaxy clusters very precisely. (Here’s my galaxy cluster primer, written as a podcast for 365 Days of Astronomy.) Their study, as with a major chunk of my thesis work, was intended to pin down the effect of dark energy—cosmic acceleration—on galaxy cluster formation and evolution. In particular, if dark energy’s effects change over time, that would have a profound influence on the number and size of galaxy clusters that form in a given era. To get a handle on this, we need a detailed census of clusters, dating back to the earliest times.

A new survey of galaxy clusters marks the beginning of a promising effort to map the birth and growth of galaxy clusters back to relatively early times. Jeeseon Song and colleagues used optical and infrared telescopes to measure the distances of 158 bright clusters in a large patch of the southern sky, looking back in time to when the Universe was less than one-third its current age. These observations provide the beginnings of a history of galaxy cluster evolution, which should help constrain models of dark energy. [Read more…]

Tracking dark energy using galaxy clusters

The test rig for DECam, which I saw when I visited Fermilab in May.

I had the privilege of visiting Fermilab in May, as part of my research for my book-in-progress. While I was there, I got to see the test rig for the Dark Energy Camera (DECam), which looks like something from Stargate or the wormhole entrance from Contact. Unfortunately for me, the camera itself had already been shipped to Chile, but yesterday DECam released its first images to the public. Here’s my story, written for Ars Technica:

DECam is mounted on the Victor M. Blanco telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile, where dark energy was first observed in 1998. As the name indicates, it is a camera, albeit a far more sensitive one than is available to consumers. The business end of the camera is a set of 62 charged-coupled devices (CCDs), yielding images of 570 megapixels. [Read more….]

The Dark Energy Camera takes its first images