We have a new member of our household - latest
developments here.
Since our atmosphere messes up most electromagnetic waves coming from space
(the main exceptions being radio waves and visible light), the advent of
satellites has revolutionized our ability to photograph the Universe in
microwaves, infrared light, ultraviolet light, X-rays and gamma rays. New
low-temperature detectors have greatly improved what can be done from the
ground as well, and the the computer revolution has enabled us to gather
and process huge data quantities, doing research that would have been unthinkable
twenty years ago. This data avalanche has transformed cosmology from being
a mainly theoretical field, occasionally ridiculed as speculative and flaky,
into a data-driven quantitative field where competing theories can be tested
with ever-increasing precision. I find CMB, LSS, lensing, SN 1a, LyAF, clusters and BBN
to be very exciting
areas, since they are all being transformed by new high-precision measurements
as described below. Since each of them measures different but related aspects
of the Universe, they both complement each other and allow lots of cross-checks.
In our standard cosmological model, the Universe was once in an extremely
dense and hot state, where things were essentially the same everywhere
in space, with only tiny fluctuations (at the level of 0.00001)
in the density. As the Universe expanded and cooled, gravitational instability
caused these these fluctuations to grow into the galaxies and the large-scale
structure that we observe in the Universe today. To calculate the details
of this, we need to know about a dozen numbers, so-called cosmological
parameters. Most of these parameters specify the cosmic matter budget, i.e.,
what the density of the Universe is made up of - the amounts of the following ingredients:
One of my main current interests is using the avalanche of new data to raise the ambition level beyond cosmological parameters, testing rather than assuming the underlying physics. My battle cry is published here with nuts and bolts details here and here.
Photos of the cosmic microwave background (CMB) radiation like the one to the left
show us the most distant object we can see: a hot, opaque wall of glowing hydrogen plasma about
14 billion light years away.
Why is it there? Well, as we look further away, we're seeing things that happened longer ago, since
it's taken the light a long time to get here. We see the Sun as it was eight minutes ago,
the Andromeda galaxy the way it was a few million years ago and this glowing surface as
it was just 400,000 years after the Big Bang. We can see that far back since the hydrogen gas that
fills intergalactic space is transparent, but we can't see further, since earlier the
hydrogen was so hot that it was an ionized plasma, opaque to light, looking like
a hot glowing wall just like the surface of the Sun.
The detailed patterns of hotter and colder spots on this wall constitute a goldmine of
information about the cosmological parameters mentioned above.
If you are a newcomer and want an introduction to CMB
fluctuations and what we can learn from them, I've written a review
here. If you don't have a physics background,
I recommend the on-line tutorials by
Wayne Hu and
Ned Wright.
If you already work on CMB, visit my
experiment compilation or
my data analysis center.
CMB experiments have already revolutionized
cosmology, but I think the best is yet to come. For instance,
NASA's
MAP satellite
will publicly release measurements of unprecedented
quality in December 2002.
Two new promising CMB fronts are opening up
--- CMB polarization and arcminute scale CMB, and are likely to keep the CMB field
lively for another decade.
Large-scale structure:
3D mapping of the Universe with galaxy redshift surveys offers another window on
dark matter properties, through its gravitational effects on galaxy clustering.
This field is currently being transformed by the
2dF Galaxy Redshift Survey
and the
Sloan Digital Sky Survey
(SDSS). The SDSS, where I am part of the large-scale structure
analysis team, will finish mapping a million galaxies in the nearby Universe over the next few years,
and complementary surveys such as DEEP and VIRMOS will map high redshifts and
the evolution of clustering.
The abundance of galaxy clusters, the largest gravitationally bound and equilibrated
blobs of stuff in the Universe, is a very sensitive probe of both the cosmic expansion history
and the growth of matter clustering. Many powerful cluster finding techniques are contributing
to rapid growth in the number of known clusters and our knowledge of their properties: identifying
them in 3D galaxy surveys, seeing their hot gas as hot spots in X-ray maps or cold spots
in microwave maps (the so-called SZ-effect) or spotting their gravitational effects with
gravitational lensing.
Yet another probe of dark matter is offered by gravitational lensing, whereby
its gravitational pull bends light rays and distorts images of distant objects.
The first large-scale detections of this effect were reported
by four groups
(astro-ph/0002500,
0003008,
0003014,
0003338)
in the year 2000, and
I anticipate making heavy use of such measurements as they continue to improve,
partly in collaboration with
Bhuvnesh Jain here at Penn.
Lensing is ultimately
as promising as CMB
and is free from the murky bias issues
plaguing LSS and LyAF measurements, since it probes the matter density directly via
its gravitational pull. I've also
dabbled some
in the stronger lensing effects caused by galaxy cores,
which offer additional insights into the detailed nature of the
dark matter.
Supernovae 1a:
If a white dwarf (the corpse of a burned-out low-mass star like our Sun) orbits another dying
star, it may gradually steal its gas and exceed the maximum mass with which it can be stable.
This makes it collapse under its own weight and blow up in a cataclysmic explosion called
a supernova of type Ia. Since all of these cosmic bombs weigh the same when they go off
(about 1.4 solar masses, the so-called Chandrasekhar mass), they all release roughly
the same amount of energy - and a more detailed calibration of this energy is possible by measuring
how fast it dims, making it the best "standard candle" visible at cosmological distances.
The supernova cosmology project
and the
high z
SN search team
mapped out how bright SN 1a looked at different redshifts found the first evidence in 1998 that
the expansion of the Universe was accelerating.
This approach can ultimately provide a direct
measurement of the
density of the Universe as a function of time,
helping unravel the nature of dark energy - I hope the
SNAP project gets funded.
The image to the left resulted from a different
type of supernova, but I couldn't resist showing it anyway...
The so-called Lyman Alpha Forest, cosmic gas clouds backlit by quasars, offers yet
another new and exciting probe of how dark has clumped ordinary matter together, and is sensitive
to an epoch when the Universe was merely 10-20% of
its present age. Although relating the measured absorption to the densities of
gas and dark matter involves
some complications,
it completely circumvents the Pandora's of galaxy biasing.
Cosmic observations are rapidly advancing on many other fronts as well,
e.g., with direct measurements of the cosmic expansion rate and the cosmic baryon fraction.
One of the main
challenges in modern cosmology is to quantify how small density fluctuations
at the recombination epoch at redshift around z=1000 evolved into the galaxies
and the large-scale structure we observe in the universe today. My Ph.D.
thesis with Joe Silk focused on ways of probing the interesting intermediate
epoch. The emphasis was on the role played by non-linear feedback, where
a small fraction of matter forming luminous objects such as stars or QSO's
can inject enough energy into their surrounding to radically alter subsequent
events. We know that the intergalactic medium (IGM)
was reionized at some point, but the details of when and how this occurred
remain open. The absence of a Gunn-Peterson trough in the spectra of high-redshift
quasars suggests that it happened before z=5, which could be achieved through
supernova driven winds from early galaxies.
Photoionization was thought to be able to
partially reionize the IGM much earlier, perhaps early enough to
affect the cosmic microwave background (CMB) fluctuations, especially in
an open universe. However, extremely early
reionization is ruled out by the COBE FIRAS constraints on the Compton
y-distortion. To make predictions for when the first objects formed and how big they were,
you need to worry about something I hate: molecules.
Although I was so fed up with rate discrepancies in the molecule literature
that I verged on making myself a Ghostbuster-style
T-shirt reading "MOLECULES - JUST SAY NO", the
irony is that my
molecule paper that I hated so much ended up being one of my most
cited ones. Whereas others that I had lots of fun with went largely unnoticed...
Like most
everybody else, I'm mystified and intrigued by the origin of gamma-ray
bursts. Applying some of my power-spectrum related data analysis techniques
to the new BATSE 3B data set, I have helped sharpen previous upper limits
on anisotropy on all angular scales as well as
tighten the previous best limits on burst repetition.
Since these new limits were quite difficult to accommodate in models with
a galactic halo origin, I firmly believed that gamma-ray bursts originated
at cosmologically large distances from us - and I'm glad that I believed
this May 1997, when the halo camp finally
conceded defeat! (Absorption lines with redshift 0.8 were detected in the
afterglow of a gamma-ray burst.)
I have a side interest
in quantum decoherence - if you'd like to know more about what this is,
check out my recent article in with John Archibald Wheeler
in Scientific American here.
I'm interested in decoherence both
for its quantitative implications for quantum computing etc
and for its philosophical implications for the interpretation
of quantum mechanics. Since macroscopic systems are virtually impossible
to isolate from their surroundings, a number of quantitative predictions
can be made for how their wavefunction will appear
to collapse, in good agreement with what we in fact observe. Similar
quantitative predictions can be made for models of heat
baths, showing how the effects of the environment cause the familiar
entropy increase and apparent directionality of time. Intriguingly, decoherence
can also be shown to produce generalized coherent
states, indicating that these are not merely a useful approximation,
but indeed a type of quantum states that we should expect nature to be
full of. All these changes in the quantum density matrix can in principle
be measured experimentally, with phases and all.
Every time I've
written ten mainstream papers, I allow myself to indulge in writing one
wacky one, like my
Scientific American article about
parallel universes.
If you don't mind really crazy ideas, check out my bananas
theory of everything. This includes
musings on the dimensionality of space and time
and on the universe containing virtually no information.
If things anthropic make you foam at the mouth,
try this.
You might enjoy this trialog
if you're interested in the question of life, the universe and everything without the equations.
Here's info about
how you will die.
Read about the
stinkymeat project or the
tragic death of
Tycho
Brahe
or experience Fred's
virtual day. Or better yet; write me an email
by clicking .
