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A GOLDEN AGE OF
ASTRONOMY
- giant telescopes, space missions
and invisible wavelengths
Michael Rowan-Robinson
Imperial College London
Sept 27th 2008
Thessaloniki
50 years of the new astronomy
The past 50 years has seen a transformation of astronomy from
the study of visible radiation with optical telescopes, to the study
of all electromagnetic radiation, with the emergence of first radioastronomy, then X-ray astronomy, ultraviolet, gamma-ray,
infrared and submillimetre astronomy - the invisible wavelengths.
This development has been made possible by a combination of
detector advances and the construction of giant telescopes both
on the ground, often on high mountain-tops, and in space.
With these advances has come a new understanding of stars and
galaxies, and of the universe we inhabit. And a host of exotic
new phenomena - quasars, pulsars, black holes, exoplanets.
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Thessaloniki
the birth of modern cosmology
Our modern picture of the universe can be said to date from 1929,
when Edwin Hubble discovered the expansion of the universe.
This breakthrough, like many of the major advances in our
understanding of the universe, including the very latest discovery
of ‘dark energy’ (see later), can be connected to advances in our
ability to measure distance in the universe.
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First steps on the distance ladder
Aristotle (384-322 BC)
- estimated the size of the earth
(+ Eratosthenes, Poseidonius, 10%)
Hipparcos (2nd C BC)
- estimated distance of the moon
(59 RE, cf modern value 60.3)
and tried to estimate the distance
of the sun
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Thessaloniki
Aristotle, by Raphael
The Copernican revolution
Copernicus (1473-1543)
- gave the correct relative
distances of the sun and planets
- absolute value not
determined accurately till the
19th century
- stars had to be much further
away than for earth-centred
model (Aristarchos, Archimedes)
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Thessaloniki
The first steps outside the solar
system
Bessel 1838
- discovered parallax of nearby star 61 Cyg, its change in
apparent direction on the sky due to the earth’s orbit round
the sun (the final proof of the Copernican system)
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The key modern distance indicator –
Cepheid variable stars
Delta Cephei is the prototype of the
Cepheid variable stars, massive stars
which pulsate and vary their light output
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Henrietta Leavitt’s breakthrough
In 1912, Henrietta Leavitt, working
at the Harvard Observatory, discovered
from her studies of Cepheids in the
Small Magellanic Cloud that the period
of Cepheid variability was related to
luminosity
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Thessaloniki
The distances of
the galaxies
In 1924 Edwin Hubble used
Leavitt’s discovery to estimate
the distance of the Andromeda
Nebula. It clearly lay far
outside the Milky Way
System.
This opened up the idea of a
universe of galaxies.
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Thessaloniki
The Palomar 5-m
telescope
Hubble used the Mount
Wilson100-inch telescope
and, later, the
Mount Palomar 200-inch
telescope, shown here.
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Thessaloniki
The expansion of the universe
Five years later he announced, based on distances to 18
galaxies, that the more distant a galaxy, the faster it is moving
away from us
velocity/distance = constant, Ho
(the Hubble law)
This is just what would be expected in an expanding universe.
The Russian mathematician Alexander Friedmann had
shown that expanding universe models are what would be
expected according to Einstein’s General Theory of Relativity, if
the universe is homogeneous (everyone sees the same picture)
and isotropic (the same in every direction).
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first detection of
electromagnetic radiation
outside the optical band:
Herschel (1800) detected
infrared radiation from the sun
Atmospheric transmission
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Radio astronomy
•1933 Karl Jansky, detected Milky Way at
radio wavelengths
•1940s Grote Reber, mapped the Milky Way
• 1945, John Hey, discovered point sources
• 1955-65 Cambridge, Parkes, surveyed the
sky and catalogued extragalactic radio
sources - radio-galaxies and quasars
• 1967 discovery of pulsars
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X-ray astronomy from space
•1948 T.R.Burnight detects X-rays from sun using V2
•1962 Giacconi detects X-ray binary Sco-X1 using Aerobee
rocket
•1963 Boyer detects Crab Nebula in X-rays (rocket)
•1965 first extragalactic X-ray source (M87, Byram, rocket)
•1970 Uhuru X-ray satellite maps sky at 2-20 KeV
many subsequent X-ray
missions, through to Chandra
and XMM, both launched in
1999 (Paul Nandra and
Ioannis Georgantopoulos will
talk about these)
Sept 27th 2008
The Uhuru
satellite
before launch
Thessaloniki
Uhuru detected X-rays
from compact sources
in binary systems (white
dwarfs, neutron stars,
black holes), from
quasars (massive black
holes) and from very hot
gas in clusters of
galaxies (100 million
degree)
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IRAS
1983 saw the launch of
IRAS, the Infrared
Astronomical Satellite,
which made the first
all-sky survey at
infrared wavelengths,
from 10-100 microns
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The launch of IRAS
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IRAS - the infrared ‘cirrus’
emission from clouds
of interstellar dust in
our Galaxy
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south celestial pole
IRAS - star forming regions
LMC, the Large Magellanic Cloud
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constellation Orion
Uultraluminous
infrared galaxies
IRAS discovered
ultraluminous infrared
galaxies, forming stars
100-1000 times faster
than our Galaxy, probably
caused by mergers between
two galaxies
this is an image of Arp 220
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IRAS - dust debris disks
IRAS also discovered dust debris disks around stars, confirmed by
imaging with the Hubble Space Telescope, evidence for planetary
systems in formation. Today over 150 exoplanets are known.
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IRAS
the IRAS all-sky survey of infrared point-sources: white: star-forming
regions, blue: red giant stars, green: galaxies
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the William Herschel Telescope on La Palma,
used to follow up IRAS galaxies
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Mapping the Universe
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3-D map of the
galaxy distribution,
constructed from
IRAS galaxy
redshift survey
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Large scale structure
The 3-dimensional
distribution of
galaxies shows
structure on
different scales.
This can be used
to estimate the
average density
of the universe.
In dimenionless
units:
Wo ~ 0.27
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Thessaloniki
The Hubble Space Telescope
(1989) Key Program
Following the first HST
servicing mission, which
fixed the telescope
aberration, a large
amount of HST
observing time was
dedicated to measuring
Cepheids in distant
galaxies, to try to
measure the Hubble
constant accurately.
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Some of the galaxies studied by
the Hubble Space Telescope
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The HST Key program final
result
Ho = 72 km/s/Mpc
uncertainty 10%
(Freedman 2001)
log V
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Implications of the Hubble
constant
Ho is (velocity/distance) so has the dimensions of (1/time).
1/Ho is the expansion age of the universe (how old the
Universe would be if no forces acting) = 13.6 billion yrs
For simplest model universe with only gravity acting, age of
universe would be 9.1 billion years (gravity slows expansion)
Sept 27th 2008
Thessaloniki
The age of the universe
We can use the colours and
brightnesses of the stars in
globular clusters to estimate
the age of our Galaxy
~ 12 billion years
Long-lived radioactive isotopes
give a similar answer
Allowing time for our Galaxy to
form, the age of the universe is
~ 13 billion years
Sept 27th 2008
Thessaloniki
The age of the universe problem
• This is a problem for the simplest models, where
gravity slows down the expansion
• To get consistency between the HST Key Program
value of Ho and the observed age of the universe,
we need to reverse the deceleration of the universe
• Something is pushing the galaxies apart
Sept 27th 2008
Thessaloniki
The discovery of the Cosmic
Microwave Background, 1965
The discovery of the Cosmic Microwave Background (CMB) by
Penzias and Wilson in 1965, and the confirmation of its blackbody
spectrum by COBE in 1991, showed that we live in a hot Big
Bang universe, dominated by radiation in its early stages.
Sept 27th 2008
Thessaloniki
How much matter is there in the
universe ?
The light elements D, He, Li
are generated from nuclear
reactions about 1 minute
after the Big Bang. The
abundances turn out to
depend sensitively on the
density of ordinary matter
in the universe.
density ~ 4.10-28 kg/cu m
Wb ~ 0.04
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Thessaloniki
Evidence for Dark Matter
the speed at which stars
orbit round a galaxy points
to the existence of a halo
of dark matter.
sensitive surveys show
that this can not be due to
stars, or gas.
Sept 27th 2008
Thessaloniki
Evidence for Dark Matter 2
images of clusters
of galaxies with
HST show arcs
due to gravitational
lensing. These can
be used to weigh
the cluster. Again,
the cluster is
dominated by dark
matter.
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Abell 2218
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Need for Dark Matter
So there is far more matter (Wo ~ 0.27 ) out
there than can be accounted for by the stuff we
are made of (Wb ~ 0.04).
85% of the matter in the universe is ‘dark’
matter (the neutralino ?)
Particle Physicists hope to detect this at the
Large Hadron Collider (switch-on Sept 10th)
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Kitt Peak, Arizona, 1974, my first observing run
JCMT
1987
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first submillimetre
survey of the sky,
using JCMT.
several very
luminous galaxies
found - galaxies
in the midst of their
main star and heavy
element formation.
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the NASA Great Observatories
HST
1990
CHANDRA
1999
COMPTON
1991
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SPITZER
2003
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SPITZER, 2003
LMC
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IC1396, the Elephant’s Trunk
- a dark globule inside
an emission nebula
- a pair of newly
formed stars have
created a cavity
- the animation shows
how the appearance
changes from the
optical, where dust
absorbs light to the
infrared where the dust
radiates
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Thessaloniki
QuickTime™ and a
MPEG-4 Video decompressor
are needed to see this picture.
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infrared emission from debris along a comet orbit
Sept 27th 2008
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visible (HST) and infrared (Spitzer) images of M51,
the ‘Whirlpool’ galaxy
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Sombrero galaxy
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Two interacting galaxies
Sept 27th 2008
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Visible and infrared images of the star-forming
galaxy Messier 82
Sept 27th 2008
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Supernovae as
Standard candles
Type Ia supernovae (explosion
of a white dwarf star in a binary
system) seem to be remarkably
uniform in their light curves.
They behave like
‘standard candles’ and can be
used to estimate distances.
Sept 27th 2008
Thessaloniki
Distant Type Ia supernovae
Recently a breakthrough in search techniques,
using 4-m telescopes to locate new supernovae, and
8-m telescopes plus the Hubble Space Telescope to
follow them up, has resulted in the detection
of Type Ia supernovae at huge distances.
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Keck and VLT
(Very Large
Telescopes)
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examples of Supernovae
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Evidence for dark energy
Over 100 Type Ia
supernova have been
found at redshifts 0.5-1.5
Comparing these to nearby
supernova, we find that in
cosmological models with
matter only, the distant
supernovae are fainter than
expected for their redshift
(Perlmutter 2002).
‘Dark energy’ is pushing the
galaxies apart.
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Thessaloniki
redshift, or distance
What is Dark Energy ?
According to Einstein’s General Theory of Relativity,
there can be an extra term in the equation for
gravity, which on large scales turns gravity into a
repulsive force (the ‘cosmological repulsion’)
This extra term, denoted L, behaves like the energy density
of the vacuum, hence ‘dark energy’
So far there is no particle physics explanation for this
dark energy
Sept 27th 2008
Thessaloniki
Mapping the Cosmic
Microwave Background
The CMB is incredibly smooth, to one part in 100,000, but the
very small fluctuations, or ‘ripples’, first mapped by the COBE
mission, are the precursors of the structure we see today.
They also tell us about the matter and energy present in the
early universe (Andrew Jaffe will say more about this)
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Thessaloniki
History of the universe
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Origin of the universe
there are speculations about the origin of the universe
theoretical physicists are trying to unify gravitation (ie General Relativity) and
quantum theory into a single unified ‘theory of everything’
current favourite is ‘string theory’, but so far this makes no predictions about
the observed universe, instead we have the ‘string landscape’
one popular idea is ‘chaotic inflation’ - our universe arose out of a vacuum
fluctuation in an infinite fluctuating void
in this picture there might be many parallel universes, each with different
properties - the ‘multiverse’
currently no evidence to support this idea, or the ‘anthropic principle’, which is
supposed to select which type of universe we find ourselves in
Sept 27th 2008
Thessaloniki
Fate of the universe
if the current consensus model, with a dominant role for dark energy, is
correct, the fate of the universe is a bleak one
the distances between galaxies will increase at an ever-accelerating rate, but
the horizon will remain fixed at more or less its current size, 13 billion light yrs
eventually, after 100 billion years, our Galaxy will have merged with
Andromeda and our other neighbours in the Local Group into a single large
and dying galaxy
there will be no other galaxies within our observable horizon
eventually all star formation will cease, all stars will die, black holes will
evaporate, and finally protons and neutrons will decay
as the Greek poet Sappho put it:
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‘nothing will remain of us’
Thessaloniki
The unanswerable questions
• Is the universe spatially finite or infinite ?
- there is a horizon defined by how far
light has travelled since the Big Bang
• What was there before the Big Bang ?
-our theories break down before we can
extrapolate to the Big Bang itself
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near future European Space Agency
missions
Herschel Mar 2009
Planck Mar 2009
GAIA Dec 2011
BepiColumbo Aug 2013
JWST June 2013 (jointly
with the US)
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how to detect z = 10 galaxies ?
James Webb Space Telescope
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ALMA, 2010
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