Posted on by Fraser Cain

How Fast Do Black Holes Spin?

How Fast Do Black Holes Spin?

There is nothing in the Universe more awe inspiring or mysterious than a black hole. Because of their massive gravity and ability to absorb even light, they defy our attempts to understand them. All their secrets hide behind the veil of the event horizon.

What do they look like? We don’t know. They absorb all the radiation they emit. How big are they? Do they have a size, or could they be infinitely dense? We just don’t know. But there are a few things we can know. Like how massive they are, and how fast they’re spinning.

Wait, what? Spinning?

Consider the massive star that came before the black hole. It was formed from a solar nebula, gaining its rotation by averaging out the momentum of all the individual particles in the cloud. As mutual gravity pulled the star together, through the conservation of angular momentum it rotated more rapidly. When a star becomes a black hole, it still has all that mass, but now compressed down into an infinitesimally smaller space. And to conserve that angular momentum, the black hole’s rate of rotation speeds up… a lot.The entire history of everything the black hole ever consumed, averaged down to a single number: the spin rate.

If the black hole could shrink down to an infinitely small size, you would think that the spin rate might increase to infinity too. But black holes have a speed limit.

“There is a speed limit to the spin of a black hole. It’s sort of set by the faster a black hole spins, the smaller is its event horizon.”

That’s Dr. Mark Morris, a professor of astronomy at UCLA. He has devoted much of his time to researching the mysteries of black holes.

“There is this region, called the ergosphere between the event horizon and another boundary, outside. The ergosphere is a very interesting region outside the event horizon in which a variety of interesting effects can occur.”

Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. Image credit: NASA/JPL-Caltech
Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. Image credit: NASA/JPL-Caltech

Imagine the event horizon of a black hole as a sphere in space, and then surrounding this black hole is the ergosphere. The faster the black hole spins, the more this ergosphere flattens out.

“The speed limit is set by the event horizon, eventually, at a high enough spin, reaches the singularity. You can’t have what’s called a naked singularity. You can’t have a singularity exposed to the rest of the Universe. That would mean that the singularity itself could emit energy or light and somebody outside could actually see it. And that can’t happen. That’s the physical limitation of how fast it can spin. Physicists use units for angular momentum that are cast in terms of mass, which is a curious thing, and the speed limit can be described as the angular momentum equals the mass of the black hole, and that sets the speed limit.”

Just imagine. The black hole spins up to the point that it’s just about to reveal itself. But that’s impossible. The laws of physics won’t let it spin any faster. And here’s the amazing part. Astronomers have actually detected supermassive black holes spinning at the limits predicted by these theories.

One black hole, at the heart of galaxy NGC 1365 is turning at 84% the speed of light. It has reached the cosmic speed limit, and can’t spin any faster without revealing its singularity.

The Universe is a crazy place.

Posted on by Brian Koberlein

Planck “Star” to Arise From Black Holes?

Artistic view of a radiating black hole. Credit: NASA

A new paper has been posted on the arxiv (a repository of research preprints) introducing the idea of a Planck star arising from a black hole.  These hypothetical objects wouldn’t be a star in the traditional sense, but rather the light emitted when a black hole dies at the hands of Hawking radiation.  The paper hasn’t been peer reviewed, but it presents an interesting idea and a possible observational test.

When a large star reaches the end of its life, it explodes as a supernova, which can cause its core to collapse into a black hole.  In the traditional model of a black hole, the material collapses down into an infinitesimal volume known as a singularity.  Of course this doesn’t take into account quantum theory.

Although we don’t have a complete theory of quantum gravity, we do know a few things.  One is that black holes shouldn’t last forever.  Because of quantum fluctuations near the event horizon of a black hole, a black hole will emit Hawking radiation.  As a result, a black hole will gradually lose mass as it radiates.  The amount of Hawking radiation it emits is inversely proportional to its size, so as the black hole gets smaller it will emit more and more Hawking radiation until it finally radiates completely away.

Because black holes don’t last forever, this has led Stephen Hawking and others to propose that black holes don’t have an event horizon, but rather an apparent horizon.  This would mean the material within a black hole would not collapse into a singularity, which is where this new paper comes in.

Diagram showing how matter approaches Planck density. Credit: Carlo Rovelli and Francesca Vidotto
Diagram showing how matter approaches Planck density. Credit: Carlo Rovelli and Francesca Vidotto

The authors propose that rather than collapsing into a singularity, the matter within a black hole will collapse until it is about a trillionth of a meter in size.  At that point its density would be on the order of the Planck density.  When the the black hole ends its life, this “Planck star” would be revealed.  Because this “star” would be at the Planck density, it would radiate at a specific wavelength of gamma rays.  So if they exist, a gamma ray telescope should be able to observe them.

Just to be clear, this is still pretty speculative.  So far there isn’t any observational evidence that such a Planck star exists.  It is, however, an interesting solution to the paradoxical side of black holes.

 

Posted on by Brian Koberlein

Black Holes No More? Not Quite.

Where is the Nearest Black Hole
Artist concept of matter swirling around a black hole. (NASA/Dana Berry/SkyWorks Digital)

Nature News has announced that there are no black holes.  This claim is made by none other than Stephen Hawking, so does this mean black holes are no more?  It depends on whether Hawking’s new idea is right, and on what you mean be a black hole.  The claim is based on a new paper by Hawking  that argues the event horizon of a black hole doesn’t exist.

The event horizon of a black hole is basically the point of no return when approaching a black hole.  In Einstein’s theory of general relativity, the event horizon is where space and time are so warped by gravity that you can never escape.  Cross the event horizon and you can only move inward, never outward.  The problem with a one-way event horizon is that it leads to what is known as the information paradox.

Professor Stephen Hawking during a zero-gravity flight. Image credit: Zero G.
Professor Stephen Hawking during a zero-gravity flight. Image credit: Zero G.

The information paradox has its origin in thermodynamics, specifically the second law of thermodynamics.  In its simplest form it can be summarized as “heat flows from hot objects to cold objects”.  But the law is more useful when it is expressed in terms of entropy.  In this way it is stated as “the entropy of a system can never decrease.”  Many people interpret entropy as the level of disorder in a system, or the unusable part of a system.  That would mean things must always become less useful over time.  But entropy is really about the level of information you need to describe a system.  An ordered system (say, marbles evenly spaced in a grid) is easy to describe because the objects have simple relations to each other.  On the other hand, a disordered system (marbles randomly scattered) take more information to describe, because there isn’t a simple pattern to them.  So when the second law says that entropy can never decrease, it is say that the physical information of a system cannot decrease.  In other words, information cannot be destroyed.

The problem with event horizons is that you could toss an object (with a great deal of entropy) into a black hole, and the entropy would simply go away.  In other words, the entropy of the universe would get smaller, which would violate the second law of thermodynamics.  Of course this doesn’t take into account quantum effects, specifically what is known as Hawking radiation, which Stephen Hawking first proposed in 1974.

The original idea of Hawking radiation stems from the uncertainty principle in quantum theory.  In quantum theory there are limits to what can be known about an object.  For example, you cannot know an object’s exact energy.  Because of this uncertainty, the energy of a system can fluctuate spontaneously, so long as its average remains constant.  What Hawking demonstrated is that near the event horizon of a black hole pairs of particles can appear, where one particle becomes trapped within the event horizon (reducing the black holes mass slightly) while the other can escape as radiation (carrying away a bit of the black hole’s energy).

Hawking radiation near an event horizon. Credit: NAU.
Hawking radiation near an event horizon. Credit: NAU.

Because these quantum particles appear in pairs, they are “entangled” (connected in a quantum way).  This doesn’t matter much, unless you want Hawking radiation to radiate the information contained within the black hole.  In Hawking’s original formulation, the particles appeared randomly, so the radiation emanating from the black hole was purely random.  Thus Hawking radiation would not allow you to recover any trapped information.

To allow Hawking radiation to carry information out of the black hole, the entangled connection between particle pairs must be broken at the event horizon, so that the escaping particle can instead be entangled with the information-carrying matter within the black hole.  This breaking of the original entanglement would make the escaping particles appear as an intense “firewall” at the surface of the event horizon.  This would mean that anything falling toward the black hole wouldn’t make it into the black hole.  Instead it would be vaporized by Hawking radiation when it reached the event horizon.  It would seem then that either the physical information of an object is lost when it falls into a black hole (information paradox) or objects are vaporized before entering a black hole (firewall paradox).

In this new paper, Hawking proposes a different approach.  He argues that rather than instead of gravity warping space and time into an event horizon, the quantum fluctuations of Hawking radiation create a layer turbulence in that region.  So instead of a sharp event horizon, a black hole would have an apparent horizon that looks like an event horizon, but allows information to leak out.  Hawking argues that the turbulence would be so great that the information leaving a black hole would be so scrambled that it is effectively irrecoverable.

If Stephen Hawking is right, then it could solve the information/firewall paradox that has plagued theoretical physics.  Black holes would still exist in the astrophysics sense (the one in the center of our galaxy isn’t going anywhere) but they would lack event horizons.  It should be stressed that Hawking’s paper hasn’t been peer reviewed, and it is a bit lacking on details.  It is more of a presentation of an idea rather than a detailed solution to the paradox.  Further research will be needed to determine if this idea is the solution we’ve been looking for.

Posted on by Fraser Cain

Weekly Space Hangout – January 10, 2014: Wake Up, Rosetta! & Top Stories from AAS

Host: Fraser Cain
Guests: David Dickinson, Amy Shira Teitel, Scott Lewis, Brian Koberlein, special guest Ruth McAvinia from the ESA

This week’s stories:
Ruth:
Wake up, Rosetta!
Facebook link to contest

David:
AAS-Gamma Ray Gravitational Lens
AAS-Death by Black Hole
Antares Launch
Remote Deployment of Cubesats
Venus at Inferior Conjunction

Scott:
Learning tools for visually impaired:
More information on the 3-D Hubble images can be found here
Here’s the press release for the iBook being released
Frontier Fields

Brian:
New Triple Star System

Amy:
ISS Life extension

Fraser:
Space Ship 2’s first Supersonic Flight

We record the Weekly Space Hangout every Friday at 12:00 pm Pacific / 3:00 pm Eastern. You can watch us live on Google+, Universe Today, or the Universe Today YouTube page.

Posted on by Fraser Cain

What is a Pulsar?

What is a Pulsar?

They are what is known as the “lighthouses” of the universe – rotating neutron stars that emit a focused beam of electromagnetic radiation that is only visible if you’re standing in it’s path. Known as pulsars, these stellar relics get their name because of the way their emissions appear to be “pulsating” out into space.

Not only are these ancient stellar objects very fascinating and awesome to behold, they are very useful to astronomers as well. This is due to the fact that they have regular rotational periods, which produces a very precise internal in its pulses – ranging from milliseconds to seconds.

Description:

Pulsars are types of neutron stars; the dead relics of massive stars. What sets pulsars apart from regular neutron stars is that they’re highly magnetized, and rotating at enormous speeds. Astronomers detect them by the radio pulses they emit at regular intervals.

An artist’s impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA / Goddard Space Flight Center / Dana Berry

Formation:

The formation of a pulsar is very similar to the creation of a neutron star. When a massive star with 4 to 8 times the mass of our Sun dies, it detonates as a supernova. The outer layers are blasted off into space, and the inner core contracts down with its gravity. The gravitational pressure is so strong that it overcomes the bonds that keep atoms apart.

Electrons and protons are crushed together by gravity to form neutrons. The gravity on the surface of a neutron star is about 2 x 1011 the force of gravity on Earth. So, the most massive stars detonate as supernovae, and can explode or collapse into black holes. If they’re less massive, like our Sun, they blast away their outer layers and then slowly cool down as white dwarfs.

But for stars between 1.4 and 3.2 times the mass of the Sun, they may still become supernovae, but they just don’t have enough mass to make a black hole. These medium mass objects end their lives as neutron stars, and some of these can become pulsars or magnetars. When these stars collapse, they maintain their angular momentum.

But with a much smaller size, their rotational speed increases dramatically, spinning many times a second. This relatively tiny, super dense object, emits a powerful blast of radiation along its magnetic field lines, although this beam of radiation doesn’t necessarily line up with it’s axis of rotation. So, pulsars are simply rotating neutron stars.

And so, from here on Earth, when astronomers detect an intense beam of radio emissions several times a second, as it rotates around like a lighthouse beam – this is a pulsar.

History:

The first pulsar was discovered in 1967 by Jocelyn Bell Burnell and Antony Hewis, and it surprised the scientific community by the regular radio emissions it transmitted. They detected a mysterious radio emission coming from a fixed point in the sky that peaked every 1.33 seconds. These emissions were so regular that some astronomers thought it might be evidence of communications from an intelligent civilization.

Although Burnell and Hewis were certain it had a natural origin, they named it LGM-1, which stands for “little green men”, and subsequent discoveries have helped astronomers discover the true nature of these strange objects.

Astronomers theorized that they were rapidly rotating neutron stars, and this was further supported by the discovery of a pulsar with a very short period (33-millisecond) in the Crab nebula. There have been a total of 1600 found so far, and the fastest discovered emits 716 pulses a second.

Later on, pulsars were found in binary systems, which helped to confirm Einstein’s theory of general relativity. And in 1982, a pulsar was found with a rotation period of just 1.6 microseconds. In fact, the first extrasolar planets ever discovered were found orbiting a pulsar – of course, it wouldn’t be a very habitable place.

Interesting Facts:

When a pulsar first forms, it has the most energy and fastest rotational speed. As it releases electromagnetic power through its beams, it gradually slows down. Within 10 to 100 million years, it slows to the point that its beams shut off and the pulsar becomes quiet.

When they are active, they spin with such uncanny regularity that they’re used as timers by astronomers. In fact, it is said that certain types of pulsars rival atomic clocks in their accuracy in keeping time.

Pulsars also help us search for gravitational waves, probe the interstellar medium, and even find extrasolar planets in orbit. In fact, the first extrasolar planets were discovered around a pulsar in 1992, when astronomers Aleksander Wolszczan and Dale Frail announced the discovery of a multi-planet planetary system around PSR B1257+12 – a millisecond pulsar now known to have two extrasolar planets.

Artist's impression of the planets orbiting PSR B1257+12. Credit: NASA/JPL-Caltech/R. Hurt (SSC)
Artist’s impression of the planets orbiting PSR B1257+12. Credit: NASA/JPL-Caltech/R. Hurt (SSC)

It has even been proposed that spacecraft could use them as beacons to help navigate around the Solar System. On NASA’s Voyager spacecraft, there are maps that show the direction of the Sun to 14 pulsars in our region. If aliens wanted to find our home planet, they couldn’t ask for a more accurate map.

We have written many articles about stars here on Universe Today. Here’s an article about a newly discovered gamma ray pulsar, and here’s an article about how millisecond pulsars spin so fast.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Posted on by Fraser Cain

Weekly Space Hangout – November 8, 2013

Host: Fraser Cain
Guests: Thad Szabo, Scott Lewis, Ian O’Neill, Alan Boyle, Nancy Atkinson, David Dickinson, Jason Major, Matthew Francis, Nicole Gugliucci

LINKS:
(Check out the comments for some more excellent discussion!)
Alan Boyle on Virgin Galactic
Nancy Atkinson on the hybrid solar eclipse
Jason Major on baby supermassive black holes
Ian O’Neill on quasars (BLACK HOLES DON’T SUCK)
Alan Boyle on Chelyabinsk
Security cam of Chelyabinsk
Matthew Francis on LUX
Nancy Atkinson on the sun (not literally)
Scott Lewis on Frontier Fields
Matthew Francis on Earth-density exoplanet
David Dickinson on GOCE
Kepler Orrery
Fireball FAQs

Posted on by Jason Major

Early Supermassive Black Holes First Formed as Twins

Two nascent black holes formed by the collapse of an early supergiant star. From a visualization by by Christian Reisswig (Caltech).

It’s one of the puzzles of cosmology and stellar evolution: how did supermassive black holes get so… well, supermassive… in the early Universe, when seemingly not enough time had yet passed for them to accumulate their mass through steady accretion processes alone? It takes a while to eat up a billion solar masses’ worth of matter, even with a healthy appetite and lots within gravitational reach. But yet there they are: monster black holes are common within some of the most distant galaxies, flaunting their precocious growth even as the Universe was just celebrating its one billionth birthday.

Now, recent findings by researchers at Caltech suggest that these ancient SMBs were formed by the death of certain types of primordial giant stars, exotic stellar dinosaurs that grew large and died young. During their violent collapse not just one but two black holes are formed, each gathering its own mass before eventually combining together into a single supermassive monster.

Watch a simulation and find out more about how this happens below:

From a Caltech news article by Jessica Stoller-Conrad:

To investigate the origins of young supermassive black holes, Christian Reisswig, NASA Einstein Postdoctoral Fellow in Astrophysics at Caltech and Christian Ott, assistant professor of theoretical astrophysics, turned to a model involving supermassive stars. These giant, rather exotic stars are hypothesized to have existed for just a brief time in the early Universe.

Read more: How Do Black Holes Get Super Massive?

Unlike ordinary stars, supermassive stars are stabilized against gravity mostly by their own photon radiation. In a very massive star, photon radiation—the outward flux of photons that is generated due to the star’s very high interior temperatures—pushes gas from the star outward in opposition to the gravitational force that pulls the gas back in.

During its life, a supermassive star slowly cools due to energy loss through the emission of photon radiation. As the star cools, it becomes more compact, and its central density slowly increases. This process lasts for a couple of million years until the star has reached sufficient compactness for gravitational instability to set in and for the star to start collapsing gravitationally.

Previous studies predicted that when supermassive stars collapse, they maintain a spherical shape that possibly becomes flattened due to rapid rotation. This shape is called an axisymmetric configuration. Incorporating the fact that very rapidly spinning stars are prone to tiny perturbations, Reisswig and his colleagues predicted that these perturbations could cause the stars to deviate into non-axisymmetric shapes during the collapse. Such initially tiny perturbations would grow rapidly, ultimately causing the gas inside the collapsing star to clump and to form high-density fragments.

“The growth of black holes to supermassive scales in the young universe seems only possible if the ‘seed’ mass of the collapsing object was already sufficiently large.”

– Christian Reisswig, NASA Einstein Postdoctoral Fellow at Caltech

Composite image from Chandra and Hubble showing supermassive black holes in the early Universe.
Composite image from Chandra and Hubble showing supermassive black holes in the early Universe.

These fragments would orbit the center of the star and become increasingly dense as they picked up matter during the collapse; they would also increase in temperature. And then, Reisswig says, “an interesting effect kicks in.” At sufficiently high temperatures, there would be enough energy available to match up electrons and their antiparticles, or positrons, into what are known as electron-positron pairs. The creation of electron-positron pairs would cause a loss of pressure, further accelerating the collapse; as a result, the two orbiting fragments would ultimately become so dense that a black hole could form at each clump. The pair of black holes might then spiral around one another before merging to become one large black hole.

“This is a new finding,” Reisswig says. “Nobody has ever predicted that a single collapsing star could produce a pair of black holes that then merge.”

These findings were published in Physical Review Letters the week of October 11. Source: Caltech news article by Jessica Stoller-Conrad.

Posted on by Nancy Atkinson

Webcast: What Happens When You Fall Into a Black Hole?

An illustration of one of the zany metaphors about the black hole firewall paradox. Credit: Maki Naro via Txchnologist.

What happens if you fall into a black hole? According to Einstein’s general theory of relativity, the fall would be uneventful, until at some point the force of gravity would rip you apart. But a new theory suggests a different fate — and if correct, could challenge our understanding of gravity and how the universe works. Join the folks from the Kavli Foundation today, September 25, at 19:00 UTC (3 pm EDT, Noon PDT) as they host a live discussion and Q & A session about the latest theories about matter entering a black hole, and how these ideas are prompting researchers to reconsider our understanding of gravity.

They’ll be discussing the “blackhole firewall paradox” that you may have been hearing about lately.

You can watch live below. To submit questions ahead of time or during the webcast, send an email to [email protected] or post on Twitter with hashtag #KavliLive.

This fun graphic above refers to the recent article written by Dennis Overbye of the New York Times, “A Black Hole Mystery Wrapped in a Firewall Paradox.” The graphic was done by illustrator Maki Naro, sent to us via the Txchnologist blog’s Zany Science Metaphors.

You can see more information about the webcast at the Kavli Foundation website.

The panelists for the discussion includes Raphael Bousso (U.C. Berkeley), Juan Maldacena (Princeton University), Joseph Polchinski (Kavli Institute for Theoretical Physics at U.C. Santa Barbara), and Leonard Susskind (Stanford University).

Posted on by Fraser Cain

Weekly Space Hangout – September 20, 2013: Antares Launch, Rocket Armadillo, ISON Craziness

It’s Friday so it’s space hangout time. Join Universe Today publisher Fraser Cain and a crew of space journalists as they discuss the big stories of the week. We’ve got the launch of the Antares rocket, a freaked out armadillo, an unexpected end to Deep Impact, ISON conspiracy madness, and more. We were joined by our regulars, but it was Elizabeth Howell’s first time. She’s been a long-time contributor to Universe Today, but this was the first time she’s joined the Weekly Space Hangout.

Host: Fraser Cain

Commentary: Amy Shira Teitel, David Dickinson, Elizabeth Howell, Jason Major

Antares Launches to the Space Station
Antares Freaks Out Armadillo
Ending for Deep Impact
More ISON Craziness
No Methane on Mars
Did the Universe Come From a Black Hole
I Didn’t Think He’d Drown
Rubber Room Under the Launch Pad

We record the Weekly Space Hangout every Friday afternoon at 12:00 Pacific, 3:00 Eastern, or 20:00 GMT. You can watch it live on Google+ or on Universe Today. You can also get the audio version within the 365 Days of Astronomy Podcast.