Transcript The Cosmological Distance Ladder

The Cosmological Distance
Ladder: the key to
understanding the universe
Michael Rowan-Robinson
Imperial College
Understanding our universe
our understanding of the universe we inhabit has always been
intimately connected with our ability to measure distance
this was true for the Greeks, and it is true of the most recent
discoveries based on fluctuations in the cosmic microwave
background radiation, which is the relic of the hot Big Bang
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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)
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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
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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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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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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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The history of the Hubble
constant
Hubble’s estimate of the Ho, the
Hubble constant, was 500
km/s/Mpc, which gave an age
for the universe of only 2 billion
years. This was soon shown to
be shorter than the age of the
earth.
From 1927 to 2001 the value of
the Hubble constant was a
matter of fierce controversy.
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The cosmological distance
ladder
Astronomers have
used a ladder of
distance estimators
to reach out to the
distant galaxies.
These have often
given inconsistent
results.
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The Hubble Space Telescope
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%
(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)
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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
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The age of the universe
problem

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

Uncertainties in Ho are
- (1) distance of Large Magellanic Cloud,
- (2) corrections for dust extinction,
- (3) corrections for local flow
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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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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.
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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
Dublin
Search for Dark Matter
The most likely candidate for dark
matter is the neutralino, a particle
predicted in ‘supersymmetric’
theories
Several searches are under way
world-wide, including this UK
experiment at Boulby Potash mine
(Imperial, Rutherford Lab)
Some anomalous events found, but
probably not the neutralino
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Mapping the Universe
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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
Wo ~ 0.27
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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 (Wo ~ 0.04).
90% of the matter in the universe is ‘dark’
matter (the neutralino ?)
Particle Physicists hope to detect this at the
Large Hadron Collider (switch-on later this year)
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Tycho Brahe’s supernova
Tycho Brahe
observed a
supernova in
Casseiopeia
in 1572.
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NRAO
NASA/Chandra
Supernova 1987A
The nearest
supernova
of modern
times
- supernova
1987A
in the Large
Magellanic
Cloud
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The Large Magellanic Cloud: a
satellite of the Milky Way
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Supernovae as
Standard candles
Type Ia supernovae (explosion
of white dwarf star) seem to be
remarkably uniform in their
light curves. They behave like
‘standard candles’ and can be
used to estimate distances.
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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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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.
(2002)
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The Fate of the Universe
Will the mutual gravitational attraction
of galaxies & clusters be sufficient to
slow down the expansion of the
Universe enough to make it contract to
a `Big Crunch’?
Or will it expand for ever?
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WM = 0
Mean distance
between galaxies
Open
WM < 1
WM = 1
Closed
fainter
- 14
-9 -7
Aug
11thyears
2008
billion
Time
today
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WM > 1
Galaxies are further from us than the
simple decelerating models, with just
gravity acting, would predict:
the deceleration is slowing.
The Universe is accelerating!!
What causes the acceleration?
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What is causing the Universe to
accelerate?
Dark Energy
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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
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The discovery of the Cosmic
Microwave Background
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
BangAuguniverse,
dominated by radiation
in its early stages.
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History of the universe
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the deepest image of the universe, the
Hubble Deep field, with galaxies seen only 2
billion years after the Big Bang. Today many
of the objects in this image would have
merged into a single big galaxy
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Mapping the Cosmic
Microwave Background (CMB)
• The CMB is incredibly smooth, to one part in 100,000, but
the very small fluctuations, or ‘ripples’, are the precursors of
the structure we see today.
They also tell us about the matter and energy present in the
early universe.
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What the CMB structure tells
us
The most prevalent scale
in the structure is literally
an echo of the ‘Bang’
(the acoustic horizon).
The angular scale of this
peak tells us that the
universe is close to being
spatially flat. In General
Relativity, this implies
Wo + L ~ 1
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angular diameter distance test
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courtesy: Paniez Paykari
The current cosmological
consensus
• Type Ia supernova need L ~ 0.73
• Large scale structure needs
Wo ~ 0.27
• CMB structure needs Wo+L ~ 1
- ( all these with uncertainty of 0.05)
• so we seem to have a consensus !
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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 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
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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’
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Prospects for the immediate
future
A future European Space Agency
mission to map the Cosmic
Microwave Background,
PLANCK Surveyor, due for
launch in December this year,
will determine cosmological
parameters with exquisite
accuracy.
PLANCK
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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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