the cosmological distance ladder: the key to understanding the universe michael rowan-robinson...

The Cosmological Distance The Cosmological Distance Ladder: the key to Ladder: the key to

understanding the universeunderstanding the universe

Michael Rowan-RobinsonMichael Rowan-Robinson

Imperial CollegeImperial College

Aug 11th 2008 Dublin

Understanding our Understanding our universeuniverse

our understanding of the universe we inhabit has always beenour understanding of the universe we inhabit has always been intimately connected with our ability to measure distanceintimately connected with our ability to measure distance

this was true for the Greeks, and it is true of the most recentthis was true for the Greeks, and it is true of the most recent discoveries based on fluctuations in the cosmic microwavediscoveries based on fluctuations in the cosmic microwave background radiation, which is the relic of the hot Big Bangbackground radiation, which is the relic of the hot Big Bang


First steps on the distance First steps on the distance ladderladder

Aristotle (384-322 BC)Aristotle (384-322 BC)

- estimated the size of the earth- estimated the size of the earth(+ Eratosthenes, Poseidonius, 10%)(+ Eratosthenes, Poseidonius, 10%)

Hipparcos (2Hipparcos (2ndnd C BC) C BC)

- estimated distance of the moon- estimated distance of the moon(59 R(59 REE, cf modern value 60.3), cf modern value 60.3)

Aristotle, by Raphael


The Copernican revolutionThe Copernican revolutionCopernicus (1473-1543)Copernicus (1473-1543)

- gave the correct relative - gave the correct relative distances of the sun and distances of the sun and planetsplanets

- absolute value not - absolute value not determined accurately till determined accurately till the 19the 19thth century century

- stars had to be much - stars had to be much further away than for earth-further away than for earth-centred modelcentred model


The first steps outside the The first steps outside the solar systemsolar system

Bessel 1838Bessel 1838

- discovered parallax of nearby star 61 Cyg, its change in - discovered parallax of nearby star 61 Cyg, its change in apparent direction on the sky due to the earth’s orbit round apparent direction on the sky due to the earth’s orbit round the sun (the final proof of the Copernican system)the sun (the final proof of the Copernican system)


The key modern distance The key modern distance indicator – Cepheid variable indicator – Cepheid variable

starsstars

Delta Cephei is the prototype of theDelta Cephei is the prototype of theCepheid variable stars, massive starsCepheid variable stars, massive starswhich pulsate and vary their light outputwhich pulsate and vary their light output


Henrietta Leavitt’s Henrietta Leavitt’s breakthroughbreakthrough

In 1912, Henrietta Leavitt, workingIn 1912, Henrietta Leavitt, workingat the Harvard Observatory, discoveredat the Harvard Observatory, discoveredfrom her studies of Cepheids in the from her studies of Cepheids in the Small Magellanic Cloud that the periodSmall Magellanic Cloud that the periodof Cepheid variability was related toof Cepheid variability was related toluminosityluminosity


The distances The distances of the galaxiesof the galaxies

In 1924 Edwin Hubble usedIn 1924 Edwin Hubble usedLeavitt’s discovery to Leavitt’s discovery to estimateestimatethe distance of the the distance of the AndromedaAndromedaNebula. It clearly lay far Nebula. It clearly lay far outside the Milky Way outside the Milky Way SystemSystem.

This opened up the idea of This opened up the idea of a universe of galaxies.a universe of galaxies.


The expansion of the The expansion of the universeuniverse

Five years later he announced, based on distances to Five years later he announced, based on distances to 18 galaxies, that the more distant a galaxy, the faster it 18 galaxies, that the more distant a galaxy, the faster it is moving away from usis moving away from us

velocity/distance = constant, Hvelocity/distance = constant, Ho o (the Hubble (the Hubble law)law)

This is just what would be expected in an expanding This is just what would be expected in an expanding universe.universe.

The Russian mathematician Alexander Friedmann The Russian mathematician Alexander Friedmann had shown that expanding universe models are what had shown that expanding universe models are what would be expected according to Einstein’s General would be expected according to Einstein’s General Theory of Relativity, if the universe is homogeneous Theory of Relativity, if the universe is homogeneous (everyone sees the same picture) and isotropic (the (everyone sees the same picture) and isotropic (the same in every direction).same in every direction).


The history of the Hubble The history of the Hubble constantconstant

Hubble’s estimate of the HHubble’s estimate of the Hoo, , the Hubble constant, was the Hubble constant, was 500 km/s/Mpc, which gave 500 km/s/Mpc, which gave an age for the universe of an age for the universe of only 2 billion years. This only 2 billion years. This was soon shown to be was soon shown to be shorter than the age of the shorter than the age of the earth. earth.

From 1927 to 2001 the From 1927 to 2001 the value of the Hubble value of the Hubble constant was a matter of constant was a matter of fierce controversy.fierce controversy.


The cosmological distance The cosmological distance ladderladder

Astronomers haveAstronomers haveused a ladder ofused a ladder ofdistance estimatorsdistance estimatorsto reach out to theto reach out to thedistant galaxies.distant galaxies.

These have oftenThese have oftengiven inconsistentgiven inconsistentresults.results.


The Hubble Space The Hubble Space Telescope Key ProgramTelescope Key Program

Following the first Following the first HSTHSTservicing mission, servicing mission, whichwhichfixed the telescopefixed the telescopeaberration, a largeaberration, a largeamount of HST amount of HST observing time was observing time was dedicated to dedicated to measuringmeasuringCepheids in distant Cepheids in distant galaxies, to try to galaxies, to try to measure the Hubblemeasure the Hubbleconstant accurately.constant accurately.


Some of the galaxies Some of the galaxies studied by the Hubble studied by the Hubble

Space TelescopeSpace Telescope


The HST Key program final The HST Key program final resultresult

log V

HHoo = 72 km/s/Mpc = 72 km/s/Mpc

uncertainty 10%uncertainty 10%

(2001)(2001)


Implications of the Hubble Implications of the Hubble constantconstant

HHoo is (velocity/distance) so has the dimensions of (1/time). is (velocity/distance) so has the dimensions of (1/time).

1/H1/Hoo is the expansion age of the universe (how old the is the expansion age of the universe (how old the Universe would be if no forces acting) = 13.6 billion yrsUniverse would be if no forces acting) = 13.6 billion yrs

For simplest model universe with only gravity acting, age ofFor simplest model universe with only gravity acting, age ofuniverse would be 9.1 billion years (gravity slows expansion)universe would be 9.1 billion years (gravity slows expansion)


The age of the universeThe age of the universeWe can use the colours andWe can use the colours andbrightnesses of the stars inbrightnesses of the stars inglobular clusters to estimateglobular clusters to estimatethe age of our Galaxythe age of our Galaxy ~ 12 billion years~ 12 billion years

Long-lived radioactive isotopesLong-lived radioactive isotopesgive a similar answergive a similar answer

Allowing time for our Galaxy toAllowing time for our Galaxy toform, the age of the universe isform, the age of the universe is ~ 13 billion years~ 13 billion years


The age of the universe The age of the universe problemproblem

A problem for the simplest models, where gravity A problem for the simplest models, where gravity slows down the expansionslows down the expansion

To get consistency between the HST Key Program To get consistency between the HST Key Program value of Hvalue of Hoo and the observed age of the universe, and the observed age of the universe, we need to reverse the deceleration of the we need to reverse the deceleration of the universeuniverse

Uncertainties in HUncertainties in Hoo are are - (1) distance of Large Magellanic Cloud, - (1) distance of Large Magellanic Cloud, - (2) corrections for dust extinction, - (2) corrections for dust extinction, - (3) corrections for local flow- (3) corrections for local flow


How much matter is there How much matter is there in the universe ?in the universe ?

The light elements D, He, LiThe light elements D, He, Li are generated from nuclearare generated from nuclear reactions about 1 minutereactions about 1 minute after the Big Bang. Theafter the Big Bang. The abundances turn out to abundances turn out to depend sensitively on thedepend sensitively on the density of ordinary matterdensity of ordinary matter in the universe.in the universe.

density ~ 4.10density ~ 4.10-28 -28 kg/cu m kg/cu m bb ~ 0.04 ~ 0.04


Evidence for Dark MatterEvidence for Dark MatterThe speed at which starsThe speed at which starsorbit round a galaxy pointsorbit round a galaxy pointsto the existence of a haloto the existence of a haloof dark matter. of dark matter. Sensitive surveys showSensitive surveys showthat this can not be due to that this can not be due to stars, or gas.stars, or gas.


Evidence for Dark Matter 2Evidence for Dark Matter 2Images of clustersImages of clustersof galaxies withof galaxies withHST show arcsHST show arcsdue to gravitationaldue to gravitationallensing. These canlensing. These canbe used to weighbe used to weighthe cluster. Again,the cluster. Again,the cluster isthe cluster isdominated by darkdominated by darkmatter.matter.

Abell 2218


Search for Dark MatterSearch for Dark MatterThe most likely candidate for The most likely candidate for darkdark matter is the neutralino, a matter is the neutralino, a particleparticle predicted in ‘supersymmetric’ predicted in ‘supersymmetric’ theoriestheories

Several searches are under waySeveral searches are under way world-wide, including this UKworld-wide, including this UK experiment at Boulby Potash experiment at Boulby Potash minemine (Imperial, Rutherford Lab)(Imperial, Rutherford Lab)

Some anomalous events found, Some anomalous events found, butbut probably not the neutralinoprobably not the neutralino


Mapping the UniverseMapping the Universe


Large scale structureLarge scale structureThe 3-dimensionalThe 3-dimensional distribution ofdistribution of galaxies showsgalaxies shows structure on structure on different scales.different scales.

This can be usedThis can be used to estimate theto estimate the average density average density of the universeof the universe

~ 0.27 ~ 0.27


Need for Dark MatterNeed for Dark Matter

So there is far more matter (So there is far more matter (~ 0.27 ) ~ 0.27 ) out there than can be accounted for by out there than can be accounted for by the stuff we are made of (the stuff we are made of (~ 0.04).~ 0.04).

90% of the matter in the universe is 90% of the matter in the universe is ‘dark’ matter (the neutralino ?)‘dark’ matter (the neutralino ?)

Particle Physicists hope to detect this at Particle Physicists hope to detect this at the Large Hadron Collider (switch-on later the Large Hadron Collider (switch-on later this year)this year)


Tycho Brahe’s supernovaTycho Brahe’s supernova

NRAO NASA/Chandra

Tycho Brahe observed a supernova in Casseiopeia in 1572.


Supernova 1987ASupernova 1987A

The nearestThe nearestsupernovasupernovaof modern of modern timestimes

- supernova supernova 1987A1987A in the Largein the Large MagellanicMagellanic CloudCloud


The Large Magellanic Cloud: a satellite of the Milky Way


Supernovae as Supernovae as Standard candlesStandard candles

Type Ia supernovae (explosionType Ia supernovae (explosionof white dwarf star) seem to beof white dwarf star) seem to beremarkably uniform in theirremarkably uniform in theirlight curves. They behave likelight curves. They behave like‘‘standard candles’ and can bestandard candles’ and can beused to estimate distances.used to estimate distances.


Distant Type Ia Distant Type Ia supernovaesupernovae

Recently a breakthrough in search techniques,Recently a breakthrough in search techniques, using 4-m telescopes to locate new using 4-m telescopes to locate new supernovae, and supernovae, and 8-m telescopes plus the Hubble Space 8-m telescopes plus the Hubble Space Telescope to Telescope to follow them up, has resulted in the detectionfollow them up, has resulted in the detection of Type Ia supernovae at huge distances.of Type Ia supernovae at huge distances.


examples of Supernovaeexamples of Supernovae


Evidence for dark energyEvidence for dark energyOver 100 Type IaOver 100 Type Ia supernova have beensupernova have been found at redshifts 0.5-1.5found at redshifts 0.5-1.5Comparing these to nearbyComparing these to nearby supernova, we find that insupernova, we find that in cosmological models withcosmological models with matter only, the distant matter only, the distant supernovae are fainter than supernovae are fainter than expected for their redshift.expected for their redshift.(2002)(2002)

redshiftredshift


Will the mutual gravitational Will the mutual gravitational attraction of galaxies & clusters attraction of galaxies & clusters be sufficient to slow down the be sufficient to slow down the

expansion of the Universe enough expansion of the Universe enough to make it contract to a `Big to make it contract to a `Big

Crunch’? Crunch’?

Or will it expand for ever?Or will it expand for ever?

The Fate of the Universe


Mean distance between galaxies

today

fainter

M = 1

Time

Closed M > 1

Open M < 1

M = 0

- 14 - 9 - 7

billion years


Galaxies are further from us Galaxies are further from us than the simple decelerating than the simple decelerating

models, with just gravity acting, models, with just gravity acting, would predict:would predict:

the deceleration is slowing.the deceleration is slowing.

The Universe is accelerating!!The Universe is accelerating!!

What causes the acceleration?What causes the acceleration?


What is causing the Universe What is causing the Universe to accelerate?to accelerate?

Dark Energy


What is Dark Energy ?What is Dark Energy ? According to Einstein’s General Theory of Relativity,According to Einstein’s General Theory of Relativity, there can be an extra term in the equation forthere can be an extra term in the equation for gravity, which on large scales turns gravity into agravity, which on large scales turns gravity into a repulsive force (the ‘cosmological repulsion’)repulsive force (the ‘cosmological repulsion’)

This extra term, denotedThis extra term, denoted, behaves like the energy , behaves like the energy density of the vacuum, hence ‘dark energy’density of the vacuum, hence ‘dark energy’

So far there is no particle physics explanation for thisSo far there is no particle physics explanation for this dark energydark energy


The discovery of the The discovery of the Cosmic Microwave Cosmic Microwave

BackgroundBackground

The discovery of the Cosmic Microwave Background (CMB) byThe discovery of the Cosmic Microwave Background (CMB) by

Penzias and Wilson in 1965, and the confirmation of its blackbodyPenzias and Wilson in 1965, and the confirmation of its blackbody

spectrum by COBE in 1991, showed that we live in a hot Bigspectrum by COBE in 1991, showed that we live in a hot Big

Bang universe, dominated by radiation in its early stages.Bang universe, dominated by radiation in its early stages.


History of the universeHistory of the universe


the deepest image of the universe, the the deepest image of the universe, the Hubble Deep field, with galaxies seen Hubble Deep field, with galaxies seen only 2 billion years after the Big Bang. only 2 billion years after the Big Bang. Today many of the objects in this image Today many of the objects in this image

would have merged into a single big would have merged into a single big

galaxygalaxy


• The CMB is incredibly smooth, to one part in 100,000, The CMB is incredibly smooth, to one part in 100,000, but the very small fluctuations, or ‘ripples’, are the but the very small fluctuations, or ‘ripples’, are the precursors of the structure we see today.precursors of the structure we see today.

They also tell us about the matter and energy present They also tell us about the matter and energy present in the early universe.in the early universe.

Mapping the Cosmic Mapping the Cosmic Microwave Background Microwave Background (CMB)(CMB)


What the CMB structure What the CMB structure tells ustells us

The most prevalent scaleThe most prevalent scale in the structure is literallyin the structure is literally an echo of the ‘Bang’an echo of the ‘Bang’ (the acoustic horizon).(the acoustic horizon).

The angular scale of thisThe angular scale of this peak tells us that thepeak tells us that the universe is close to beinguniverse is close to being spatially flat. In Generalspatially flat. In General Relativity, this impliesRelativity, this implies

~ 1~ 1


angular diameter distance testangular diameter distance test

courtesy: Paniez Paykari


The current cosmological The current cosmological consensusconsensus

• Type Ia supernova need Type Ia supernova need ~ ~ 0.730.73

• Large scale structure needs Large scale structure needs oo ~ 0.27 ~ 0.27

• CMB structure needs CMB structure needs oo++ ~ 1 ~ 1

- ( all these with uncertainty of - ( all these with uncertainty of 0.05)0.05)

• so we seem to have a so we seem to have a consensus !consensus !


Origin of the universeOrigin of the universe there are speculations about the origin of the universethere are speculations about the origin of the universe

theoretical physicists are trying to unify gravitation (ie General theoretical physicists are trying to unify gravitation (ie General Relativity) andRelativity) and quantum theory into a single unified ‘theory of everything’quantum theory into a single unified ‘theory of everything’

current favourite is ‘string theory’, but so far this makes no current favourite is ‘string theory’, but so far this makes no predictions aboutpredictions about the observed universe, instead have the ‘string landscape’the observed universe, instead have the ‘string landscape’

one popular idea is ‘chaotic inflation’ - our universe arose out of a one popular idea is ‘chaotic inflation’ - our universe arose out of a vacuumvacuum fluctuation in an infinite fluctuating voidfluctuation in an infinite fluctuating void

in this picture there might be many parallel universes, each with in this picture there might be many parallel universes, each with differentdifferent properties - the ‘multiverse’properties - the ‘multiverse’

currently no evidence to support this idea, or the ‘anthropic currently no evidence to support this idea, or the ‘anthropic principle’, which isprinciple’, which is supposed to select which type of universe we find ourselves insupposed to select which type of universe we find ourselves in


Fate of the universeFate of the universe if the current consensus model, with a dominant role for dark if the current consensus model, with a dominant role for dark energy, isenergy, is correct, the fate of the universe is a bleak onecorrect, the fate of the universe is a bleak one

the distances between galaxies will increase at an ever-accelerating the distances between galaxies will increase at an ever-accelerating rate, butrate, but the horizon will remain fixed at more or less its current size, 13 the horizon will remain fixed at more or less its current size, 13 billion light yrsbillion light yrs

eventually, after 100 billion years, our Galaxy will have merged witheventually, after 100 billion years, our Galaxy will have merged with Andromeda and our other neighbours in the Local Group into a single Andromeda and our other neighbours in the Local Group into a single largelarge and dying galaxyand dying galaxy

there will be no other galaxies within our observable horizonthere will be no other galaxies within our observable horizon

eventually all star formation will cease, all stars will die, black holes eventually all star formation will cease, all stars will die, black holes willwill evaporate, and finally protons and neutrons will decayevaporate, and finally protons and neutrons will decay

as the Greek poet Sappho put it: ‘nothing will remain of us’as the Greek poet Sappho put it: ‘nothing will remain of us’


Prospects for the Prospects for the immediate futureimmediate future

PLANCK

A future European Space AgencyA future European Space Agency mission to map the Cosmicmission to map the Cosmic Microwave Background,Microwave Background, PLANCK Surveyor, due for PLANCK Surveyor, due for launch in December this year,launch in December this year, will determine cosmologicalwill determine cosmological parameters with exquisite parameters with exquisite accuracy.accuracy.


The unanswerable The unanswerable questionsquestions

• Is the universe spatially finite or Is the universe spatially finite or infinite ?infinite ?

- there is a horizon defined by how there is a horizon defined by how farfar light has travelled since the Big light has travelled since the Big BangBang

• What was there before the Big What was there before the Big Bang ?Bang ?

-our theories break down before we our theories break down before we can can extrapolate to the Big Bang itselfextrapolate to the Big Bang itself

the cosmological distance ladder: the key to understanding the universe michael rowan-robinson...

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