Fall 2006 ASTR 2030 Homepage
Fall 2006 ASTR 2030 Black Holes: Weekly Summaries
- What is a black hole?
An object whose gravity is so strong that not even light can escape.
- Event horizon - Schwarzschild radius.
- The point from within which light cannot escape
- The center of the black hole, where the spacetime curvature becomes infinite
- A place where space and time as we know them come to an end
- Astronomical evidence exists for two kinds of black holes:
- Stellar-sized black holes, observed in X-ray binary systems
- Supermassive black holes, observed at the centers of galaxies
- Black hole bends light around it.
- Tidal forces tear infaller apart:
- well outside the horizon of a stellar-sized black hole
- well inside the horizon of a supermassive black hole
- Time dilation allows you to travel "faster than light"
- Relativistic effects of moving through a scene at near the speed of light:
- Aberration: ahead appears fisheyed, behind zoomed
- Color: blueshift (higher energy photons) ahead, redshift (lower energy photons) behind
- Brightness: brighter ahead, dimmer behind
- Time: faster ahead, slower behind
- Stable and unstable orbits around a black hole
- Object falling into a black hole appears to an outside observer to redshift and freeze at the horizon
- Nothing special appears happens as you yourself fall through the horizon
This was mainly a week of summary and review, in preparation for the midterm.
- Quasars are unresolved ("quasi-stellar"),
extremely bright (as much as 1000 times as luminous as a whole galaxy)
objects seen at the centers of galaxies.
Typically they have large redshifts, indicating that they are moving rapidly away from us,
which in turn indicates that they are far away.
They are thought to be powered by supermassive black holes at the centers of galaxies.
- The quasar 3C273 and its jet
- Two indications that the jet is moving at almost the speed of light:
- The jet is superluminal - blobs in the jet are observed to move several times faster than the speed of light.
- We see only one jet, the jet coming towards us, which is relativistically brightened;
there is undoubtedly a second jet going in the opposite direction, which we do not see because it is relativistically dimmed.
- History of the development of General Relativity, and Einstein's rejection of Black Holes - read Th Ch 2, 3.
It will also help you to read Th Ch 5, 6,
which document how improved understanding of cold matter (Ch 5)
and of nuclear burning (Ch 6)
eventually forced physicists to conclude that stars of sufficient mass must eventually collapse to a black hole.
- The thing that finally broke down the mental block against black holes was Finkelstein's (1958)
(re-)discovery of a falling reference frame in which there was no "Schwarzschild singularity" at the horizon.
- No hair theorem:
An isolated black hole rapidly (on few light crossing times) evolves
to a state characterized by just 3 properties:
It loses its "hair" by gravitational radiation.
- Electric charge
- Angular momentum (spin)
- Falling into a Black Hole:
- What happens when you watch someone else fall into a black hole?
- What happens when you fall into a black hole?
- The River Model of Black Holes.
- General Relativity predicts the existence of gravitational waves that move at the speed of light.
- The Schwarzschild geometry for a non-rotating, uncharged black hole
- Intervals of spacetime can be either timelike, or lightlike, or spacelike:
- Timelike = possible worldline of a particle (traveling normally, at less than the speed of light)
- Lightlike = possible worldline of light
- Spacelike = a possible "now-line": a line between two events that are simultaneous with respect to some observer
- Collapse to a black hole
- Schwarzschild wormhole, white hole
- The Postulates of General Relativity:
- Spacetime forms a 4-dimensional continuum (postulate carried over from Special Relativity).
- Existence of locally inertial frames:
In a small neighborhood of any point of spacetime,
there exist locally inertial, or free-fall, frames
with respect to which the rules of Special Relativity apply
(unaccelerated objects move in straight lines at constant velocity; the speed of light is constant).
- Strong Principle of Equivalence of gravity and acceleration (1907):
The rules of physics in a gravitating frame
are the same as those in an accelerating frame.
- Einsteins equations (1915):
- Consequences of the Principle of Equivalence:
- Gravitational redshift;
- Gravitational bending of light;
- Spacetime is curved.
This was mainly a week of summary and review, in preparation for the midterm.
- We watched "Under the Night", the first episode of the "Andromeda" series,
and did a
This week was about how to script a movie.
We watched excerpts from two movies, "Contact", and "Walt Disney's The Black Hole".
At the end of the week we did an in class
to critique Chapter 4 of Thorne as if it were a story.
A well-structured movie has 4 parts:
- The first 1/4 of the movie should set up the "World", or genre, of the story.
The story could be a cartoon, a cowboy film, an action movie, or whatever,
but once the world has been set up, the movie should remain consistent
to that world.
While setting up the "World",
the movie should introduce the hero, the villain, and any other
The movie is often more interesting if the hero has flaws
and the villain has qualities.
- During the main part of the movie, the hero should encounter Problems
of one kind or another.
A novel can develop characters by getting inside their heads
and telling you what the characters are thinking,
but that is not possible in a movie.
Movies develop characters by facing them with problems,
and seeing how the characters respond to those problems.
- The hero should Overcome the problems encountered.
Often overcoming the problems generates new problems,
so that there is a cycle between encountering problems
and overcoming them.
- The movie should build up to a Climax in its final part.
The climax should be the most exciting part of the movie.
The movie should end with closure, a resolution of some kind.
If you have a budget, spend it here.
Sometimes the movie ends with the climax;
other times there is an epilogue after the climax.
Sometimes the ending is perfect like a fairy tale,
but it can be more interesting if the ending has a twist
or is not perfect.
Mitch Begelman lectured on:
More on x-ray binaries and Active Galactic Nuclei in
Week 11 (below).
Rosalba Perna lectured on:
- What happens to cold matter when you compress it very prodigiously?
- A solid or liquid is almost incompressible
because the atoms are tightly packed together,
in a regular lattice in a solid, irregularly in a liquid.
- Atoms resist compression
because electrons satisfy an exclusion principle:
you can't pack electrons together more closely than a wavelength.
- Like any particle, an electron is both wave and particle:
For a non-relativistic electron,
momentum p equals mass times velocity,
p = m v.
- The wavelength of electrons in atoms is set by their orbital velocities.
The characteristic atomic velocity of electrons is
1/137 of the speed of light:
The (1/137) factor is the "fine-structure constant",
which describes the characteristic strength of electromagnetism.
- To compress atoms requires compacting electrons
to the point where their velocities exceed the atomic velocity c/137.
This requires a pressure exceeding the pressure at the center of Jupiter.
- Thus planets of Jupiter's mass or less
are made of incompressible liquid or solid;
this is why the moons and planets in the solar system all have about
the same density,
and more massive planets have larger radii.
- At pressures higher than the pressure at the center of Jupiter,
electrons are squeezed out of atoms,
and form a compressible electron degenerate gas.
- Thus cold objects more massive than Jupiter
are held up by electron degeneracy pressure.
They are effectively cold white dwarfs.
- An electron degenerate gas is so compressible
that larger mass white dwarfs have smaller radii.
- The white dwarf sequence terminates
when the electrons have been compressed to the point that
their velocities are relativistic (almost the speed of light).
This happens at the Chandrasekhar limit of 1.4 solar masses.
- If mass is added to a white dwarf to bring it above 1.4 solar masses,
then the white dwarf will collapse.
- A white dwarf that exceeds the Chandrasekhar limit
will collapse to a neutron star.
- A neutron star is held up in part by the degeneracy pressure of neutrons,
and in part by the strong (nuclear) force.
- The neutron star sequence terminates at an upper mass limit of about 3 solar masses.
- A neutron star whose mass exceeds about 3 solar masses
will collapse to a black hole.
A star that for a while becomes comparable in brightness to its parent galaxy.
Dick McCray's hypertext
includes chapters on
Novae and supernovae,
Neutron stars and black holes.
- 1054. Chinese astronomer Yang Wei-te reported a "guest star" (supernova) in constellation Taurus,
which was recorded in the
Annals of the Sung Dynasty.
- 1931. Fritz Zwicky points out that some novae (new stars)
are exceptionally bright. He calls them "supernovae".
He begins a solo campaign to discover supernovae.
- 1932. James Chadwick discovers the neutron.
- 1933. Baade & Zwicky
make their famous prediction that supernovae produce neutrons stars.
- 1940s. Baade and others
measure the expansion rate of Messier 1, the Crab nebula,
and show that it must have originated in an explosion around 1054.
They conclude that the Crab nebula is the remnant of the Supernova of 1054.
- 1967. Jocelyn Bell discovers a pulsar,
with a precise period of 1.337301 seconds,
then three more.
The first few pulsars are given the name "LGM" (Little Green Men).
Observations that pulsars pulse over a wide range of radio frequencies
ruled out the possibility of the source being intelligent life.
- 1968. American astronomers discover a 0.033 second pulsar in the Crab nebula.
The period is too short and too regular to be anything other than a rotating neutron star.
- Today. More than 1300 pulsars are now known in our Galaxy, with periods ranging from 0.00156 to several seconds.
A pulsar is a rotating, magnetized neutron star.
There are two kinds of supernova:
Core collapse supernovae
Core collapse supernova
Show H lines
Show no H lines
In star-forming regions, spiral arms
Anywhere in a galaxy
Young, massive star (> 8 solar masses)
Old white dwarf
- Most stars spend most of their lives fusing Hydrogen (H) to Helium (He) at their centers.
These are main sequence stars.
The Sun is a main sequence star.
- When stars exhaust H at their center,
the He core contracts, H burns in a shell around the core,
the envelope expands, and the star becomes a Red Giant.
In the Red Giant phase, the star develops a strong wind.
- For stars less massive than about 8 suns,
the wind eventually drives off the entire envelope.
For a while, the star is seen as a
Planetary Nebula (nothing to do with planets),
containing a small hot star inside a glowing gaseous nebula.
The central star cools to become a White Dwarf.
- For stars more massive than about 8 suns,
the He core burns to heavier elements.
The core develops a complicated multi-layered structure.
At the center is iron (Fe), which contains no more nuclear fuel
- Fe is the most tightly bound of all nuclei.
- The inert Fe core is held up by electron degeneracy pressure.
When its mass has increased to 1.4 solar masses, the Chandrasekhar limit,
the core collapses to a neutron star.
Electrons and protons are pressed together into neutrons,
releasing an explosion of neutrinos:
The collapsing envelope of the star bounces off the neutron star core,
producing a Supernova.
- A core-collapse supernovae is powered by the gravitational energy
released by the collapse of the core.
- Computer simulation by Adam Burrows.
- For stars more massive than about 25 or 30 suns,
material falling back on to the newly formed neutron star
takes the neutron star over its 3 solar mass limit, causing it to collapse to a black hole.
- X-ray binary systems.
- Quasars are the most extreme (luminous) form of Active Galactic Nuclei
- 3C273, the nearest bright quasar, the first quasar discovered, by Maarten Schmidt in 1963.
- Name quasar shortened from quasi-stellar-object
- an unresolved, star-like source with a non-stellar spectrum and a large redshift.
- Large redshift, from the expansion of the Universe, indicates that quasars are at cosmological distances.
- Quasars were most abundant at a redshift of 2, when the Universe was younger.
- Intensely luminous, as much as 1000 times the brightness of their parent galaxy.
- Typically variable, on timescales of days; this indicates that they are less than lightdays in size.
- Active Galactic Nuclei - a generic term for a variety of phenomena seen at the centers of galaxies
- Variable - on timescales of days to years.
- Non-stellar spectrum - radio, x-rays, gamma-rays; optical emission lines.
- Jets, often relativistic - superluminal motion, one-sided.
- Some AGN, known as blazars (after the prototype, BL Lac),
have featureless spectra variable down to hour timescales
interpreted as we are looking down the maw of a relativistic jet which is pointed directly at us,
and therefore brightened, blueshifted, and speeded up.
- John Kormendy's 2001 census of Supermassive Black Holes in Galactic Nuclei
- Observations of the centers of nearby galaxies
indicate the presence of a large unseen gravitational mass in a small region
- it must be a black hole.
- Every galaxy large enough to have a bulge appears to have a central black hole.
- Black Hole masses range from millions (106) to billions (109) of solar masses.
- M87, the huge galaxy at the center of the Virgo cluster, at the center of the Local Supercluster of galaxies,
contains the most massive black hole known in the local Universe, 3 × 109 solar masses.
- Maser emission from M106 reveals a thin, warped accretion disk around a 3.9 × 107 solar mass black hole.
- Observations of the motions of stars, both angular motions on the sky, and radial motions from spectroscopy (redshifts and blueshifts),
indicate the presence of a 4 × 106 solar mass black hole at the center of our own Galaxy, the Milky Way.
- The observational evidence that the centers of galaxies contain supermassive black holes
The most direct evidence is:
- A lot of mass (inferred from velocities of stars or gas)
- in a small space (inferred from imaging).
Supporting evidence is:
- Active Galactic Nuclei.
- Recall the no-hair theorem,
that an isolated BH is characterized by just 3 things,
its mass, charge, and spin.
Charged black holes
- The Reissner-Nordström (RN)
geometry describes the geometry of empty
space around a spherical,
electrically charged, black hole.
The RN geometry assumes that the electric charge of the black hole
is concentrated at a point at the central singularity.
- Real black holes probably have very little electric charge.
But the geometry of a charged black hole
is qualitatively similar to that of a rotating black hole,
so models of black holes often use charge as a surrogate for spin
(as in the BHFS).
- The RN geometry is, surprisingly,
gravitationally repulsive in its core.
- The gravitational repulsion causes a suite of remarkable phenomena:
the inside of the RN black hole has a wormhole
which connects to a white hole
which connects to a new universe.
The new universe connects via another
blackhole-wormhole-whitehole to yet another universe,
and so on.
- The RN geometry is however inconsistent:
it assumes that the BH is empty,
yet the repulsive core will prevent mass or charge
from falling on to the central singularity,
so in reality the BH could not be empty.
of the RN geometry
describes how space falls into the RN black hole,
turns around, and flies back out through a white hole into a new universe.
Horizons, outer and inner, occur wherever the velocity of space
hits the speed of light.
- The RN geometry proves to be violently unstable
at its inner horizon.
The RN geometry predicts that light that falls into the black hole
will pile up at the inner horizon,
producing an infinitely energetic shell,
contradicting the hypothesis that the RN geometry is empty.
An infaller passing through the inner horizon
sees this light as an infinitely bright flash of light.
The concentration of light and matter at the inner horizon
destabilizes the inner horizon,
changing the geometry drastically.
- In a real black hole,
the instability at the inner horizon
has the consequence that
the region from the inner horizon inward would contain
a dense, relativistic plasma, which would vaporize an infaller.
Rotating black holes
- The Kerr-Newman (KN) geometry
describes the geometry of empty
space around a spinning, possibly charged, black hole.
- Real black holes probably spin.
- Centrifugal force causes the KN geometry to be gravitationally repulsive in its core.
- The phenomenology of the KN geometry
is quite similar to that of the RN geometry:
the gravitational repulsion causes the KN geometry to have
both inner and outer horizons,
and blackhole-wormhole-whitehole connections to new universes.
- The KN geometry is, again, inconsistent,
because the black hole cannot be empty in its core if the core is repulsive.
- The singularity of the KN geometry forms a ring, kept open by the centrifugal force.
- The outer horizon and inner horizon are confocal ellipsoids,
with the ring singularity at the focus.
- The other side of the disk bounded by the ring singularity
is a new region, the "antiverse", at negative radius.
In the antiverse, the black hole appears to have negative mass.
- In the antiverse, circles around the axis near the disk are timelike.
These circles form "closed timelike loops" (CTLs),
where time keeps repeating itself.
- The inner horizon is, as in the charged black hole, violently unstable.
- As in a charged black hole,
the instability causes the region inside the inner horizon
to contain a dense, relativistic plasma, which would vaporize an infaller.
- You watched the "Black Holes and Relativity"
Planetarium show at Fiske.
Much of what you saw recapitulated things you have
read in the book or heard in the lectures.
However, Fiske showed you some things hard to see other
than in a planetarium.
- The x-ray sky is very different from the visible sky.
X-rays are high energy photons,
so when you see an x-ray source in the sky,
it inevitably represents some energetic and violent process going on.
- X-ray sources on the sky are distributed in two ways:
- X-ray sources in the plane of the Milky Way,
especially towards the center of the Milky Way.
Most of these are x-ray binary systems,
though a few are supernova remnants.
- X-ray sources out of the plane of the Milky Way.
These are almost all Active Galactic Nuclei.
Phew we needed that.
This week was on
- Hawking radiation is quantum mechanical radiation from black holes.
Quantum mechanically, virtual pairs of particles can pop out of the vacuum.
The black hole swallows one of the particles (which effectively carrys negative energy),
leaving the other particle (carrying positive energy) to go off to the outside world.
- Hawking radiation has a black body spectrum.
- The characteristic wavelength of Hawking radiation is roughly equal to
the diameter of the black hole.
Thus if you could see a black hole by its Hawking radiation,
it would look fuzzy, a quantum mechanical object.
- The Hawking temperature and luminosity of astronomical black holes is tiny.
The radiation observed from near astronomical black holes is emitted
by hot gas from an accretion disk, not Hawking radiation.
- The Hawking temperature and luminosity of so-called mini-black holes
is much larger.
Stephen Hawking hypothesized that mini-black holes might be created in the Big Bang,
but there is no observational evidence for their existence.
- A mini-black hole with mass less than the mass of a mountain
can evaporate in the age of the Universe.
A mini-black hole of 1000 tons will evaporate in 1 second,
in a burst of gamma rays and high energy particles.
From a human point of view this is a large explosion,
but astronomically it is a tiny explosion,
far less energetic than a nova, supernova, or gamma-ray burst.
This week was on cosmology.
Cosmology is one of the major applications of general relativity,
the other major application being black holes.
A third major application, gravitational waves,
will become important in the next two decades
with the advent of gravitational wave astronomy.
- The Standard Model of Cosmology.
- Hubble's law v = H0 d,
recession velocity = Hubble's constant H0 times distance.
- Implies that the Universe is expanding, and that there was a Big Bang.
- H0 determines the age of the Universe, t » 1/H0
(the equality is not exact because of the deceleration or acceleration of the Universe).
- The geometry of the Universe
- W = (actual density of the Universe)/(critical density of the Universe).
(W, Omega, is the last letter of the Greek alphabet).
- Through Einstein's equations of general relativity,
W determines the curvature, and also the fate, of the Universe.
- Open, flat, and closed geometries of the Universe.
- In 1998 a revolution in cosmology resulted from the observation that the expansion of
the Universe is accelerating(!)
- This means that the mass-energy density of the Universe must be dominated by some gravitationally repulsive substance,
called Dark Energy.
- Dark Energy could be the zero point energy of the quantum vacuum, but no one really understands what it is.
- The Cosmic Microwave Background (CMB) is the radiation remnant of the primeval hot Big Bang fireball.
- Observationally, the Cosmic Microwave Background:
- Has an exquisite black body spectrum, with a temperature of 2.726 Kelvin.
- Is almost uniform on the sky.
- Shows a "dipole" distortion from the motion of the Sun through the CMB, at 365 km/s.
- After subtraction of the dipole distortion,
the residual temperature fluctuations are tiny, a few parts in 105.
- The power spectrum of fluctuations shows a pattern of acoustic peaks
in remarkable agreement with the predictions of the theory of Inflation.
- Theoretically, the Cosmic Microwave Background:
- Supports the idea that there was a hot Big Bang.
- The CMB cools as the Universe expands,
the wavelengths of CMB photons being stretched (redshifted) by the expansion of the Universe.
- Its uniformity and black body spectrum tell us that the Universe used to be much simpler when it was young.
- Comes to us from the Epoch of Recombination,
when the Universe was 370,000 years old, and the temperature was 3000 Kelvin.
- At Recombination, hydrogen and other elements combined
from an ionized plasma of nuclei and free electrons
to a neutral gas of atoms.
As a result, the Universe changed from being opaque to transparent,
allowing the CMB to propagate freely to us from that time.
- The theory of Inflation postulates that the during its earliest moments
(the first 10-33 seconds or so),
the mass-energy of the Universe was dominated by vacuum energy.
Vacuum energy is gravitationally repulsive,
and would cause the Universe to expand exponentially (inflate),
doubling in radius every 10-35 seconds.
- Inflation solves all of the following problems:
- The Horizon Problem.
How come regions of the CMB more than 2° apart have the same temperature,
even though they were causally disconnected at the time of Recombination?
- If gravity is always attractive, then why is the Universe expanding?
- How come the Universe is so flat?
- How come the Universe is so large?
- Where did the mass-energy content of the present day Universe come from?
- What produced the small fluctuations that grew by gravity into galaxies and stars today?
Answer: quantum fluctuations of the vacuum.
- Inflation predicts several key observed features of fluctuations in the temperature of the CMB:
- A scale-invariant spectrum of fluctuations at large scales (much larger than the horizon size at Recombination).
- A regular sequence of acoustic peaks.
- Flat curvature.
- A random (Gaussian) noise pattern.
- Inflation ends when the vacuum energy decays into other forms of energy,
namely matter and radiation.
This is where matter and radiation in today's Universe came from.
- The vacuum energy that powers inflation is thought to be the energy of the unification of forces.
Specifically, the energy is thought to be associated with the hypothesized
unification of the electroweak and color (or strong) forces into a GUT (Grand Unified Theory) Force.
The energy scale is not accessible to present day particle accelerators,
so the nature of the unification is not well understood.
Fall 2006 ASTR 2030 Homepage
Updated 2006 Dec 15