Subatomic Particles
Described on the Web
Cosmology Web Sites
Web-Based Interface Allows Students to Create Models of the Universe
Frequently Asked Questions in Cosmology by Ned Wright, UCLA
Guide to the cosmic background radiation by Wayne Hu, U. Chicago
Microwave Anisotropy Probe (MAP)
Cosmic Background Imager
Cosmology 101 from NASA's MAP project
Particle physics joint site of several labs
The good news from NASA's Hubble Space Telescope is that Einstein was right--maybe.
A strange form of energy called "dark energy" is looking a little more like the repulsive force that Einstein theorized in an attempt to balance the universe against its own gravity. Even if Einstein turns out to be wrong, the universe's dark energy probably won't destroy the universe any sooner than about 30 billion years from now, say Hubble researchers.
"Right now we're about twice as confident than before that Einstein's cosmological constant is real, or at least dark energy does not appear to be changing fast enough (if at all) to cause an end to the universe anytime soon," says Adam Riess of the Space Telescope Science Institute, Baltimore.
Riess used Hubble to find nature's own "weapons of mass destruction" -- very distant supernovae that exploded when the universe was less than half its current age. The apparent brightness of a certain type of supernova gives cosmologists a way to measure the expansion rate of the universe at different times in the past.
Riess and his team joined efforts with the Great Observatories Origins Deep Survey (GOODS) program, the largest deep galaxy survey attempted by Hubble to date, to turn the Space Telescope into a supernova search engine on an unprecedented scale. In the process, they discovered 42 new supernovae in the GOODS area, including 6 of the 7 most distant known.
Cosmologists understand almost nothing about dark energy even though it appears to comprise about 70 percent of the universe. They are desperately seeking to uncover its two most fundamental properties: its strength and its permanence.
In a paper to be published in the Astrophysical Journal, Riess and his collaborators have made the first meaningful measurement of the second property, its permanence.
Currently, there are two leading interpretations for the dark energy, as well as many more exotic possibilities. It could be an energy percolating from empty space as Einstein's theorized "cosmological constant," an interpretation which predicts that dark energy is unchanging and of a prescribed strength.
An alternative possibility is that dark energy is associated with a changing energy field dubbed "quintessence."
This field would be causing the current acceleration -- a milder version of the inflationary episode from which the early universe emerged.
When astronomers first realized the universe was accelerating, the conventional wisdom was that it would expand forever. However, until we better understand the nature of dark energy--its properties--other scenarios for the fate of the universe are possible.
If the repulsion from dark energy is or becomes stronger than Einstein's prediction, the universe may be torn apart by a future "Big Rip," during which the universe expands so violently that first the galaxies, then the stars, then planets, and finally atoms come unglued in a catastrophic end of time. Currently this idea is very speculative, but being pursued by theorists.
At the other extreme, a variable dark energy might fade away and then flip in force such that it pulls the universe together rather then pushing it apart.
This would lead to a "big crunch" where the universe ultimately implodes. "This looks like the least likely scenario at present," says Riess.
Understanding dark energy and determining the universe's ultimate fate will require further observations. Hubble and future space telescopes capable of looking more than halfway across the universe will be needed to achieve the necessary precision. The determination of the properties of dark energy has become the key goal of astronomy and physics today.
Electronic images and additional information are available at:
National Science Foundaion Press Release, May 23, 2002
NSF PR 02-41
Astronomers operating from a remote plateau in the Chilean desert have produced the most detailed images ever made of the oldest light emitted by the universe, providing independent confirmation of controversial theories about the origin of matter and energy.
Pushing the limits of available technology, the Cosmic Background Imager (CBI) funded by the National Science Foundation (NSF) and California Institute of Technology (Caltech) detected minute variations in the cosmic microwave background, the radiation that has traveled to Earth over almost 14 billion years. A map of the fluctuations shows the first tentative seeds of matter and energy that would later evolve into clusters of hundreds of galaxies.
The measurements also provide independent evidence for the long-debated theory of inflation, which states that the universe underwent a violent expansion in its first micro-moments. After about 300,000 years it cooled enough to allow the seeds of matter to form and became "transparent," allowing light to pass through. CBI observed remnants of that early radiation. The data are also helping scientists learn more about the repulsive force called "dark energy" that appears to defy gravity and force the universe to accelerate at an ever-increasing pace.
"This is basic research at its finest and most exciting," said NSF Director Rita Colwell. "Each new image of the early universe refines our model of how it all began. Just as the universe grows and spreads, humankind's knowledge of our own origins continues to expand, thanks to the technical expertise and patient persistence of scientists such as these."
"We have seen, for the first time, the seeds that gave rise to clusters of galaxies, thus putting theories of galaxy formation on a firm observational footing," said team leader Anthony Readhead of Caltech. "These unique high-resolution observations provide a new set of critical tests of cosmology, and provide new and independent evidence that the universe is flat and is dominated by dark matter and dark energy."
Readhead, with Caltech colleagues Steve Padin and Timothy Pearson and others from Canada, Chile and the United States, generated the finest measurements to date of the cosmic microwave background. Cosmic microwave background (CMB) is a record of the first photons that escaped from the rapidly cooling, coalescing universe about 300,000 years after the cosmic explosion known as the Big Bang that is commonly believed to have given birth to the universe.
Data from the CBI on temperature distributions in the CMB support a modification of the Big Bang theory; that modification is called inflation theory. Inflation states that the hot plasma of the initial universe underwent an extreme and rapid expansion in its first 10 -32 second. The variations in temperature measured by the CBI are as small as 10 millionths of a degree.
By plotting the peaks of temperature distribution, the scientists showed that the precise CBI data are entirely consistent with inflation and confirm earlier findings by other scientists. In April 2000, an international team of cosmologists led by Caltech's Andrew Lange announced the first compelling evidence that the universe is flat-that is, its geometry is such that parallel lines will neither converge or diverge. Lange's team observed at a different frequency from CBI, using a high- altitude balloon flown over Antarctica.
Since then, two other teams -- using independent methods - have revealed their analyses of the very faint variations in temperature among the comic microwaves. The four instruments have conducted precise measurements of parameters that cosmologists have long used to describe the early universe. Each set of data has offered new clues to the form of the embryonic plasma and has drawn scientists closer to definitive answers. NSF has supported the work of all four teams and their instruments, some of them for more than 15 years.
Five papers on the CBI data were submitted to the Astrophysical Journal for publication.
The CBI consists of 13 interferometers mounted on a 6-meter diameter platform, operating at frequencies from 26 GHz to 36 GHz. Located in the driest desert in the world -- the Atacama -- CBI takes advantage of the low humidity at an altitude of 5,080 meters (16,700 feet). NSF has supported the CBI research since 1995. The National Council of Science and Technology of Chile provided the CBI site.
For more information and images, see:
http://www-dev.nsf.gov/od/lpa/news/advance/pr0241_images.htm
and
http://www.astro.caltech.edu/~tjp/CBI/
Jodrell Bank Press Release, May 23, 2002
A new picture of the early universe
Physicists from the Universities of Cambridge and Manchester and the Instituto de Astrofisica de Canarias in Tenerife have released the first results of new high-precision observations of the relic radiation from the Big Bang, often called the cosmic microwave background or CMB. These observations have been made with a novel radio telescope called the Very Small Array (VSA) situated on the Mount Teide in Tenerife. The images show the beginnings of the formation of structure in the early Universe. From the properties of the image, scientists can obtain vital information on just what happened in the early universe and distinguish between competing cosmological theories.
Radiation from the Big Bang fireball has been travelling across the universe, cooling as space expands. Today, we see the faint relic radiation in all directions on the sky at a temperature of just 3 degrees centigrade above absolute zero, giving a picture of the universe when it was less than one 50,000th of its present age. Because galaxies must have formed out of the primeval fireball, astrophysicists have predicted that they will have left imprints in the radiation. Across the sky, there should be tiny variations in the temperature of the relic radiation. However, these ripples are very weak---only one 10,000th of a degree C.
During its first year of operation the VSA has observed three patches of sky, each some 8 x 8 degrees across. It can see detail down to one third of a degree, well matched to the typical size of interesting temperature variations. The VSA has 14 aerials, each somewhat akin to a satellite TV dish but only 15 cm across. The signals from each aerial are combined, forming an interferometric array - a technique pioneered by Cambridge physicists. The array is able to filter out unwanted terrestrial and atmospheric radiation allowing the the extremely faint CMB sky signal common to all the aerials to be detected. This approach allows high precision observations to be made at modest cost - the capital cost of the VSA was 2.6 million GBP. The performance of the VSA also results from using advanced receivers built at Manchester University and from the outstanding atmospheric conditions at the 2.4 km high Teide Observatory on Tenerife. The VSA can therefore measure specific, individual structures in the relic radiation with great precision.
A small number of other experiments have made similar observations. The different experiments work in different ways and face different challenges and sources of error; a key advantage of this diversity is that if their results agree, one can be confident that they are correct. One special strength of the VSA is that it is an interferometer array; another is that it is able to robustly remove the contaminating radiation from radiogalaxies and quasars that lie between us and the CMB relic radiation. The VSA results provide amazing confirmation of the current picture of the Universe.
The VSA observations of the CMB released today reveal the following properties of our Universe:
1) The curvature of space is close to zero -- we live in a spatially 'flat' universe.
2) The material in the universe is dominated by dark matter.
3) There is direct evidence for 'vacuum dark energy' which is currently not well understood, but is causing the universe to accelerate.
4) There are multiple peaks in the CMB power spectrum. This is direct evidence that all the structure in the universe today is due to microscopic quantum-mechanical fluctuations, inflated to astronomical size in the very early universe.
Images and Web Sites
---------------------
Images and captions, and links to the scientific papers, are available from the Cavendish Laboratory website at
http://www.mrao.cam.ac.uk/telescopes/vsa
See also:
Press release in both English and Spanish at the Instituto de Astrofisica
de Canarias website:
http://www.iac.es/gabinete/noticias/2002/m05d23.htm
Press release and images at the University of Manchester JBO website:
The Jodrell Bank website:
http://www.jb.man.ac.uk/news/vsa/
The PPARC website:
http://www.pparc.ac.uk/Cnt/CM.asp
The VSA is a collaborative project between the Astrophysics Group at Cambridge University's Cavendish Laboratory, Manchester University's Jodrell Bank Observatory, and the Instituto de Astrofi'sica de Canarias (IAC) in Tenerife. The project is funded by the UK Particle Physics and Astronomy Research Council and the IAC.
NASA Press Release 02-73, April 24, 2002
Pushing the limits of its powerful vision, NASA's Hubble Space Telescope has uncovered the oldest burned-out stars in our Milky Way Galaxy. These extremely old, dim "clockwork stars" provide a completely independent reading on the age of the universe from previous methods.
The ancient white dwarf stars, as seen by Hubble, turn out to be 12 to 13 billion years old. Because earlier Hubble observations show that the first stars formed less than one billion years after the universe's birth in the big bang, finding the oldest stars puts astronomers well within arm's reach of calculating the absolute age of the universe.
Though previous Hubble research sets the age of the universe at 13 to 14 billion years based on the rate of expansion of space, the universe's birthday is such a fundamental and profound value that astronomers have long sought other age-dating techniques to cross-check their conclusions. "This new observation short-circuits getting to the age question, and offers a completely independent way of pinning down that value," says Harvey Richer of the University of British Columbia, Vancouver, Canada.
The new age-dating observations were done by Richer and colleagues by using Hubble to hunt for elusive ancient stars hidden inside a globular star cluster 7,000 light-years away in the constellation Scorpius. The results will be published in the Astrophysical Journal Letters.
Conceptually, the new age-dating observation is as elegantly simple as estimating how long ago a campfire burned by measuring the temperature of the smoldering coals. For Hubble, the "coals" are white dwarf stars, the burned-out remnants of the earliest stars in our galaxy.
Hot, dense spheres of carbon "ash" left behind by the long-dead star's nuclear furnace, white dwarfs cool down at a predictable rate -- the older the dwarf, the cooler it is, making it a perfect "clock" that has been ticking for almost as long as the universe has existed.
This approach has been recognized as more reliable than age-dating the oldest stars still burning by nuclear fusion, which relies on complex models and calculations about how a star burns its nuclear fuel and ages. White dwarfs are easier to age-date because they are simply cooling, but the trick has been finding the dimmest and hence longest- running "clocks."
As white dwarfs cool they grow fainter, and this required that Hubble train a steady gaze on the ancient globular star cluster M4 for eight days over a 67-day period. This allowed for even fainter dwarfs to become visible, until at last the coolest -- and oldest -- dwarfs were seen. These stars are so feeble (30th magnitude -- considerably fainter than originally anticipated for any Hubble telescope imaging with the original cameras), they are less than one-billionth the apparent brightness of the faintest stars that can be seen by the naked eye.
Globular clusters are the first pioneer settlers of the Milky Way. Many coalesced to build the hub of our galaxy and formed billions of years before the appearance of the Milky Way's magnificent pinwheel disk (as further confirmed by Richer's observations). Today, 150 globular clusters survive in the galactic halo. The globular cluster M4 was selected because it is the nearest to Earth, so the intrinsically feeblest white dwarfs are still apparently bright enough to be picked out by Hubble.
In 1928, Edwin Hubble's measurements of galaxies made him realize that the universe was uniformly expanding, which meant the universe had a finite age that could be estimated by mathematically "running the expansion backward." Edwin Hubble first estimated the universe was only two billion years old. Uncertainties over the true expansion rate led to a spirited debate in the late 1970s, with estimates ranging from 8 billion to 18 billion years. Estimates of the ages of the oldest normal "main-sequence" stars were at odds with the lower value, since stars could not be older than the universe itself.
In 1997 Hubble Space Telescope astronomers broke this impasse by announcing a reliable age for the universe, calculated from a very precise measurement of the expansion rate. The picture soon got more complicated when astronomers using Hubble and ground-based observatories discovered the universe was not expanding at a constant rate, but accelerating due to an unknown repulsive force termed "dark energy." When dark energy is factored into the universe's expansion history, astronomers arrive at an age for the universe of 13-14 billion years. This age is now independently verified by the ages of the "clockwork" white dwarfs measured by Hubble.
http://oposite.stsci.edu/pubinfo/pr/2002/10
PPARK Press Release (PPARK is the UK equivalent of the NSF), April 4, 2002
The cause of gamma ray bursts, the most violent and explosive events in the Universe, has remained a mystery since they were first discovered in 1967. Now a team of scientists, led by astronomers from the University of Leicester, believes they have found an answer to the puzzle. Their research results [published in 'Nature' on 4th April 2002] indicate that gamma ray bursts are caused by the death of a star so huge that when it dies in a supernova, its core collapses to form a black hole, resulting in an intense outburst of gamma rays.
Dr. Julian Osborne of the University of Leicester explained,' Until now it was unclear whether gamma ray bursts were caused by a supernova explosion of a giant star collapsing into a black hole, or by the coalescence of two neutron stars. Each event could result in an intense outburst of gamma rays, followed by an X-ray afterglow. By analysing this X-ray afterglow we believe we have determined the gamma ray burst origin.'
The scientists used the EPIC (European Photon Imaging Camera) instrument on the European Space Agency's [ESA] XMM-Newton space telescope to capture the X-ray afterglow of a recent gamma ray explosion in a galaxy 10 billion light years from Earth, and then conducted a detailed spectral analysis of the data. The results were a great surprise.
Dr. James Reeves of the University of Leicester added, 'For the first time ever traces of light chemical elements were detected - including magnesium, silicon, sulphur, argon, and calcium - neutron star collisions are not expected to make these. Also, the hot cloud containing these elements is moving towards us at one tenth of the speed of light. This suggests that the gamma ray burst resulted from the collapse of the core of a giant star following a supernova explosion. This is the only way the light elements seen by XMM-Newton, speeding away from the core, could be produced. So the source of the gamma ray burst is a supernova and not a neutron star collision.' He added, 'The confirmation by XMM-Newton that gamma ray bursts are associated with supernovae therefore brings scientists closer to understanding the process that leads to the burst itself'.
The UK has taken a leading role in XMM-Newton, as Prof. Ian Halliday, Chief Executive of the Particle Physics and Astronomy Research Council explains "From the development of the XMM-Newton concept, the UK has been taking a strong role, with the University of Leicester leading the way on the EPIC instrument and on the Survey Science Centre consortium which processes all the data recorded. The Mullard Space Science Laboratory built the Optical Monitor telescope on XMM-Newton, which also captured the fading optical signature of the gamma ray burst."
Prof. Ian Halliday, adds "These latest findings will be tested by SWIFT; a NASA led space mission that scientists from the University of Leicester and Mullard Space Science Laboratory are helping to build. Once it is launched in autumn next year [2003], SWIFT will study over 1000 gamma ray bursts, spotting them rapidly then automatically turning two much more sensitive telescopes to study these events. This means that the resulting X-rays can be studied mere seconds after the event, instead of the hours presently required to manoeuvre existing space-based telescopes with instructions from the ground."
Images
Hypernova, XMM, and Swift images are available from the PPARC web site:
http://www.pparc.ac.uk/Nw/Press/Gamma.asp
Background notes
XMM-Newton
At 3.9 tonnes and 10 metres long, the X-ray Multi-Mirror [XMM]-Newton is the
biggest and most sensitive X-ray telescope ever to be placed in to orbit. It
is able to study extremely faint X-ray sources from stars and galaxies in
the most distant parts of the Universe.
EPIC the European Photon Imaging Camera
This instrument, built by an international team of European scientists led
by Dr. Martin Turner of the University of Leicester uses silicon Charge
Coupled Devices to provide simultaneous X-ray images and spectra so that the
chemical composition of distant X-ray sources can be studied.
Gamma ray bursts
Every day, somewhere in the Universe, there is a gamma ray burst; they are
the most powerful and violent phenomena in the Universe. Gamma ray bursts
last only a minute or so. But in that
time the energy released is equivalent to the instantaneous conversion of up
to the entire Sun's mass into energy, following Einstein's famous equation
E=mc2, making gamma ray bursts second only to the Big Bang in total power.
The intense burst of gamma rays is followed by an X-ray glow that lasts a
few days; it is this X-ray afterglow that was observed by XMM-Newton about
11 hours after the gamma ray burst.
Gamma ray bursts were discovered in 1967 by the US Military VELA satellites
where they mimicked the signatures of terrestrial nuclear tests. Several
thousand have been detected since, although it was only five years ago that
they were proven to come from distant galaxies.
Supernova
A supernova is the explosion produced when a massive star ends its life; one
occurs in our own Galaxy approximately every hundred years. Seen from the
Earth, it would be initially visible in daylight and would be the brightest
star in the sky for about six months. Supernovae in distant galaxies appear
much fainter, but briefly outshine the host galaxy itself. The chemical
elements that make up the Earth, and ultimately ourselves, were formed in
supernovae.
Royal Astronomical Society Press Notice, April 3, 2002
Neutrinos, the lightest of the known elementary particles, weigh a billionth (one part in a thousand million) of a hydrogen atom at most, and can account for no more than one-fifth of the dark matter in the Universe, according to findings by astronomers in Cambridge, who used data from the Anglo-Australian telescope 2dF Galaxy Redshift Survey (2dFGRS). The results were presented by Dr Ofer Lahav of Cambridge University.
The findings come from detailed study of the 2dF (two-degree field) Galaxy Redshift Survey, compiled using the Anglo-Australian telescope in New South Wales, Australia. The telescope has created the world's largest three-dimensional catalogue of galaxies so far, currently consisting of 220,000 galaxies. A team of 30 researchers is analysing the survey to answer fundamental questions about the Universe.
Neutrinos come in three different varieties, and were long thought to have no mass at all, but observations of neutrinos emitted from the Sun and created by cosmic rays in the Earth's atmosphere have in the last few years revealed that this cannot be the case. A determination of the masses of neutrinos would provide clues about the physics of processes occurring under conditions beyond the reach of current particle physics experiments.
It has long been known that there is more to our Universe than we can see in the starry sky. Indeed, astronomers now know that the visible parts of the Universe, such as stars and galaxies, only constitute a small fraction of its total mass. Neutrinos do not interact with light, and are therefore a candidate for the mysterious invisible dark matter in the Universe. The mass of the neutrinos affects the growth of clumps that evolve into the large structures we observe in the Universe at the present epoch. Since neutrinos are very light, they move at nearly the speed of light over vast regions, smoothing out the clumpiness of the matter.
To study this effect of the tiny neutrinos on the universe, Dr Oystein Elgaroy and Dr Ofer Lahav (both from the Institute of Astronomy, University of Cambridge, UK) together with other 2dFGRS team members, compared the distribution of galaxies mapped out by the 2dFGRS with theoretical calculations of how the matter would be distributed in model universes with different values for the neutrino mass. From this confrontation of theory with observation, they were able to conclude that the neutrinos must have a mass smaller than a billionth of a hydrogen atom. They also concluded that the neutrinos make up less han 20% of the dark matter in the Universe, and that the rest therefore has to be in some as yet unknown form.
"It is fascinating that we can use enormous structures like galaxies to learn about the properties of the lightest of all the particles in the Universe," says Oystein Elgaroy.
"The dark matter problem has bothered astronomers for over 70 years. If indeed neutrinos have mass, the composition of matter and energy in the universe is even more complicated than the astronomers have so far imagined," says Ofer Lahav.
Recently a group of particle and nuclear physicists announced that they had observed a new type of nuclear decay process involving neutrinos. Their result is still being debated by scientists around the world but, as it stands, it implies that the three neutrinos have very nearly the same mass, and that its value is roughly a few parts in ten billion of the mass of a hydrogen atom.
"Our result from the galaxy survey does not rule out a neutrino mass as deduced from the particle physics experiment," says Oystein Elgaroy. The redshift surveys of millions of galaxies that will be completed in the next few years will set even tighter limits on the mass of the neutrino".
Ofer Lahav adds: "The latest cosmological data suggest that the universe is a mysteriously dark place. It is probably made of four entities, three of them rather exotic: ordinary matter, neutrinos, another form of dark matter which is 'cold' and energy (so-called 'dark energy' or vacuum energy) represented by the cosmological constant, suggested originally by Einstein".
Astronomers have observed a Dark Matter object directly for the first time. Images and spectra of a MACHO microlens - a nearby dwarf star that gravitationally focuses light from a star in another galaxy - were taken by the NASA/ESA Hubble Space Telescope and the European Southern Observatory's Very Large Telescope. The result is a strong confirmation of the theory that a large fraction of Dark Matter exists as small, faint stars in galaxies such as our Milky Way.
The Riddle of Dark Matter
The nature of Dark Matter is one of the fundamental puzzles in
astrophysics today. Observations of clusters of galaxies and the large
scale structure of individual galaxies tell us that no more than a
quarter of the total amount of matter in the Universe consists of
normal atoms and molecules that make up the familiar world around us.
Of this normal matter, no more than a quarter emits the radiation we
see from stars and hot gas. So, a large fraction of the matter in our
Universe is dark and of unknown composition.
For the past ten years, active search projects have been underway for possible candidate objects for Dark Matter. One of many possibilities is that the Dark Matter consists of weakly interacting, massive sub- atomic sized particles known as WIMPs. Alternatively Dark Matter may consist of massive compact objects (MACHOs), such as dead or dying stars (neutron stars and cool dwarf stars), black holes of various sizes or planet-sized collections of rocks and ice.
The MACHOs
In 1986, Bohdan Paczynski from Princeton University realised that if
some of the Dark Matter were in the form of MACHOs, its presence could
be detected by the gravitational influence MACHOs have on light from
distant stars. If a MACHO object in the Milky Way passes in front of a
background star in a nearby galaxy, such as the Large Magellanic Cloud,
then the gravitational field of the MACHO will bend the light from the
distant star and focus it into our telescopes. The MACHO is acting as a
gravitational lens, increasing the brightness of the background star
for the short time it takes for the MACHO to pass by. Depending on the
mass of the MACHO and its distance from Earth, this period of
brightening can last days, weeks or months. The form and duration of
the brightening caused by the MACHO - the microlensing light curve -
can be predicted by theory and searched for as a clear signal of the
presence of MACHO Dark Matter. MACHOs are described as `microlenses'
since they are much smaller than other known cases of gravitational
lensing, such as those observed around clusters of galaxies.
The MACHO Project
Astronomers from the Lawrence Livermore National Laboratory, the Center
for Particle Astrophysics in the United States and the Australian
National University joined forces to form the MACHO Project in 1991.
This team used a dedicated telescope at the Mount Stromlo Observatory
in Australia to monitor the brightness of more than 10 million stars in
the Large Magellanic Cloud over a period of eight years. The team
discovered their first gravitational lensing event in 1993 and have now
published approximately twenty instances of microlenses in the
direction of the Magellanic Clouds. These results demonstrate that
there is a population of MACHO objects in and around the Milky Way
galaxy that could comprise as much as one half of the Milky Way total
(baryonic/normal-matter) Dark Matter content.
Hubble Obtains the First-Ever Image of a MACHO
In order to learn more about each microlensing event, the MACHO team
has used Hubble to take high-resolution images of the lensed stars. One
of these images showed a faint red object within a small fraction of an
arc-second from a blue, main sequence background star in the Large
Magellanic Cloud. The image was taken by Hubble 6 years after the
original microlensing event, which had lasted approximately 100 days.
The brightness of the faint red star and its direction and separation
from the star in the Large Magellanic Cloud are completely consistent
with the values indicated 6 years earlier from the MACHO light curve
data alone. This Hubble observation further reveals that the MACHO is a
small faint, dwarf star at a distance of 600 light-years with a mass
between 5% and 10% of the mass of the Sun.
VLT Adds Spectral Information
To further confirm its findings, members of the MACHO team sent in a
special application for observing time on the FORS instrument on the
European Southern Observatory's (ESO) Very Large Telescope (VLT)) to
make spectra of the object. ESO responded swiftly and positively to the
request. Although it was not possible to separate the spectra of the
MACHO and background star, the combined spectrum showed the
unmistakable signs of the deep absorption lines of a dwarf M star
superimposed on the spectrum of the blue main sequence star in the
Large Magellanic Cloud.
The Nature of Dark Matter
The combination of the microlensing light curve from the MACHO project,
the high-resolution images from Hubble and the spectroscopy from the
VLT has established the first direct detection of a MACHO object, to be
published in the international science journal Nature on 6 December.
The astronomers now have a complete picture of the MACHO: its mass,
distance and velocity. The result greatly strengthens the argument that
a large fraction of the `normal' Dark Matter in and around our Galaxy
exists in the form of MACHOs and that this Dark Matter is not as dark
as previously believed!
Future searches for MACHO-like objects will have the potential to map out this form of Dark Matter and reach a greater understanding of the role that Dark Matter plays in the formation of galaxies. These efforts will further strengthen the drive to reveal the secrets of Dark Matter and take a large step towards closing the books on the mass budget of the Universe.
Information not only about the Hubble constant but also about gamma ray bursters, gravitational lenses, Chandra X-ray Observatory, and other interesting topics.
A Nature of the Universe Debate, in memory of David Schramm, was held in Washington,
DC, on 4 October 1998. The title was: "The Nature of the Universe: Cosmology
Solved?" Speakers were:
P. James E. Peebles, an astrophysical cosmologist's viewpoint
Michael S. Turner, a particle cosmologist's viewpoint
Owen Gingerich, introductory talk
Joseph I. Silk, introductory talk
Margaret J. Geller, moderator
Details are available at:
http://antwrp.gsfc.nasa.gov/debate/debate98.html
http://physics.stanford.edu/linde.
A direct link to his 1994 Scientific American article on inflationary cosmology is at
http://www.sciam.com/specialissues/0398cosmos/0398linde.html
See http://physics.stanford.edu/linde for movies showing the process of self-reproduction of inflationary universes, as calculated by Prof. Andrei Linde of Stanford University.
The results of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale were discussed in August 2000. Wendy Freedman, Barry Madore, and Robert Kennicutt were the Principal Investigators. In the Key Project, which took up 420 hours of Hubble time, they observed dozens of Cepheid variables in each of 18 galaxies, and used those Cepheid distances to determine the slope of Hubble's law. The value they derived for Hubble's constant is 75 km/s/Mpc, plus or minus 10%. The plus or minus 10% is the key advance. As Prof. Robert Kirshner of Harvard put it, we used to have a disparity of a factor of 2, which is like being uncertain if you have one foot or two, and now we are down to a toe.
But Allan Sandage and his coworkers still don't accept this value. They have not given up their scientific defense of values for Hubble's constant of about 60.
John Hawley of the University of Virginia and Katherine Holcomb have an algebra-only book on cosmology, "Foundations of Modern Cosmology" (Oxford University Press, copyright 1998). My colleague Karen Kwitter, who reviewed the book, says that it is a fabulous book and is very suitable for a non-majors but upper-level course on cosmology. The authors' website contains a lot of relevant information and photographs.
A slide set showing cosmic-background-radiation images is available from the Astronomical Society of the Pacific.
Recent data from the ground are available; the experiment contact I have is Max Tegmark, then of the Institute for Advanced Study, Princeton.