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PrHEP hep2001 GeV, down to the 18 10 × 4 . 2 ' 2 / 1 − ) N πG (8 International Europhysics Conference on HEP ≡ P M ) eV, well below any known scale of fundamental physics. This has 3 − (10 O From an observational perspective cosmology is today in excellent shape — ∗ [email protected] PROCEEDINGS Abstract: advances in instrumentation and data processingdetail have back enabled to us when the to first study galaxiesbackground the formed, which universe map provide in the a fluctuations measure in of thehistory cosmic the microwave reliably overall geometry, back and to reconstruct at thestudies least thermal the of primordial the nucleosynthesis era. Hubbledriven However expansion by recent some rate deep form have ofin suggested ‘dark’ Nature that (vacuum) of the energy. universe If true, is this accelerating, implies a new energy scale refocussed attention on the notoriousgeneral cosmological relativity constant problem and at quantumsituation the will field interface require theory. of fundamental modifications It to our is ideas possible about that gravity. the resolution of this In the Standard Model Lagrangian, the term corresponding to the cosmological con- Speaker. ∗ Department of Physics, 1E-mail: Keble Road, Oxford OX1 3NP, UK Last year, at thethe Osaka HEP possibility conference, that Fukugita the [1]constant evolution reviewed — of the Einstein’s the emerging “greatest universegrowing evidence blunder” today for acceptance [2]. is of being thismany In driven amazing particle the by claim intervening a physicists. period cosmological bypresent vacuum If there the poses has true, astronomical a been deep the community, mystery and implied for even any small fundamental by but theorystant [3]. non-zero is energy one densitythe of of second the the one two beingdue ‘super-renormalisable’ the to terms quadratic radiative divergence allowedexperimental corrections in by success [4]. the of the mass the gauge Tometry Standard of tame symmetries, between Model fundamental the has bosonic scalar required latter fields andscale. the sufficiently fermionic introduction Thus in fields of the order a whichcan supersym- cutoff is be to scale (softly) lowered explain from of broken the the at the Planck about Standard scale, the Model, Fermi viewed as an effective field theory, 1. Introduction Subir Sarkar New results in cosmology PrHEP hep2001 ), i.e. 4 EW minimum M ( Subir Sarkar O 1 s. In principle we know ∼ –2– 300 GeV, albeit at the expense of introducing over 150 new generally accepted solution to the problem of the discrepancy ∼ 2 no / 1 − F G ∼ s, would have left any detectable relics [7]. Thus early universe theorists 6 EW − ) corresponding to the observationally indicated vacuum energy. Thus even M s but it appears that neither this event, nor the subsequent QCD confine- P 10 12 ∼ − /M between expectations and observations. 10 2 EW 60 Given this sorry situation on the theoretical front, I will focus in this talk solely on It is generally recognised that a true resolution of the cosmological constant problem ∼ M 10 ( “halfway” down (on a logarithmicO scale) from the Planck scale towards the energy scale of contribution to the vacuum energy density from quantum fluctuations of masses and other parameters inmany the non-renormalisable Lagrangian operators (as which wellnucleon as can requiring decay generate delicate etc, flavour-changing control neutral so of currents, the as not to violate experimental bounds). This implies a presently have a free handattempting in to speculating account about for the thedark thermal known history matter relics, before and viz. the the BBN density the(LSS) era, perturbation baryon and in which asymmetry, the seeded left abundance therapid its of growth observational of imprint large-scale progress on structure beingsion the made studies cosmic on of microwave the thephysical background cosmological CMB, processes (CMB). will front, that created provide particularly Hopefully our valuable in the observable constraints universe preci- and on all (and it possibly contains. clues to) the the introduction of supersymmetry∼ cannot eradicate a discrepancy by at least a factor of ment at the physics underlying the evolutionat back to the epoch of electroweak symmetry breaking can only be achievedtheory in and a the full possibility that quantuminteresting there theory ideas exist of concerning new gravity. dimensions possibleremains in Recent true values Nature that developments of there have in is generated thereferred string many cosmological to constant above. [5]. Ofalways been course However the the it expectation cosmological thatzero. constant somehow problem we However would if is understand it not onemore is new day severe in why but since it fact there is it non-zero had of exactly also and expansion raises dynamically special? a important The cosmic today,to resolution the ‘coincidence’ the of crisis problem, construction these is viz. conundrums of even in may cosmological why well terms is models require of present modifications (and epochphenomenologically Einstein’s the satisfactory interpretation general has of relativity, been astronomical presented although data) either. to date nothe such new alternative observational developments whichevidence and is in for particular a provide cosmologicaltheoretical a activity constant. critical in assessment early For of universedark lack the cosmology matter of concerning etc. baryogenesis, time The inflation,if I most particle there exciting will are development not new in discusstion dimensions this of area in the has Nature the continuing been then earlyHowever the they universe attempts realisation may so have that to the dramatically constrain standardbrought altered lore such home the [6] new the evolu- may physics fact havebefore that from to we the cosmological be still big arguments substantially have bang have rewritten. no nucleosynthesis also detailed (BBN) understanding of era the begining early at universe International Europhysics Conference on HEP Fermi scale, PrHEP hep2001 Subir Sarkar 025 has been reported 338 [8] by constraining . . . This is a possibility that =3 =2 α z z CMB temperature measurement Inferred time variation in the fine- Kat 7 2 . . 8 +1 − 1 absolute . Figure 2: structure constant from observations of QAS. The bottom panelning shows of an the arbitrary(from raw bin- Ref.[16]). results in the upper panel =12 –3– =0 z CMB T 4 K has been made in a QAS at redshift ± Measurements of the CMB tem- =10 ). This cannot be accomodated within the “Quasi-Steady State Cosmology” in z For example such measurements also imply constraints on time-variations of funda- CMB T . (Subsequently a possible problem with this measurement has been noted but an even (1 + perature at various redshifts comparedthe with standard expectationmalised (dotted to line) the nor- COBE measurement at Figure 1: (from Ref.[8]). 2 which the CMB arisesalready through severely constrained thermalisation by of theform closeness starlight [11]. of [10] the — observed Themeasurements a CMB prospects spectrum have mechanism been to for that the discussed was constraining Planck in other detail [12]. alternative cosmologies by further such [9].) As shownBang in cosmology Figure that 1, the∝ all blackbody such temperature measurements of the agree CMB with increases the with expectation the in redshift a Big mental constants, in particular of the fine-structure constant better constrained measurement of That we live in andirectly universe with which has the beenThis measurement hotter is in of done the by a past observingabsorption has higher fine-structure systems recently transitions temperature been (QAS) between confirmed for atomic —Such levels the measurements cold of have gas CMB C already I clouds attures in been have along high quasar had possible the to redshift. for be line(i.e. considered some of as time sight upper other but to limits thanto because the distant the being alternative inferred quasars. observed excitation immersed tempera- mechanisms level in populations. a Recently Planckian an radiation bath) may have contributed International Europhysics Conference on HEP 2. The observational situation of such competing mechanisms through simultaneousH measurements of the rotational levels in PrHEP hep2001 5 6 10 − − 10 10 10 3 Gyr, × ∼ × Hubble z ± 5) . 18) Hipparcos 5 . 90% of the 5 . at 0 U, as shown Subir Sarkar ± ∼ ± 5 from zero [16]. . 238 72 α/α . 0 =(3 α/α − 2 Gyr, the estimated [27]. Based on direct − =( 2 0 α/α . 1% on ∆ 0 H α/α . ∼ inthepast[15].Asshownin 6of∆ . α =1 z 3 Gyr for the oldest stars in globular . 1 deviation: ∆ ± cosmologies [22]. 12 corresponds to an age of 12 σ . 5 . 0 α ± –4– 14 6 Gyr [24] but it decays so little over the lifetime . . 0 4 = 14 Gyr) had already been detected in similar ± 2 ± 5 Gyr) is in principle a more sensitive probe.) The / . 7 6 1 . . t 5. Moreover it is argued that correcting for any possible . the significance of the deviation of ∆ 13 3 =4 − 2 − Th ( / , as shown in Figure 5. However significantly smaller values of 5 1 1 . t 0 232 − increase ∼ U( z Mpc 238 1 − Key Project has presented its final results on 7kms ± 3 ± are obtained by ‘physical’ methods e.g. measurements of time delays in gravitationally =72 0 0 in Figure 3 [23]. (In fact measurements of the distancesusing to these 18 to nearby calibrate spiral fiveH secondary galaxies methods, (using they Cepheid find variables) that and all data are consistent with stars and used to inferoftheUniversethat an age of 15 derived abundance, log(U/H)= H age of the universe;Figure each 2, sample the optical yields sample a of smaller 49 QAS value shows of a 4 systematic effects would 2.1 The age of theReturning universe to the standarddirect cosmology, another measurement recent of observational the highlightof age has our of been Galaxy an the extremely through metal-poor detection (i.e. of the very 385.957 old) nm star in line the of halo singly ionized [20]. Moreover considerations of[21] BBN and place this an is upper hard limit to of reconcile with varying- over the redshift range International Europhysics Conference on HEP has attracted much theoreticalresult interest has [13, been 14] and claimedoptical very [15, data recently sets 16]. a and positive two Four 21 observational independent cm samples and mm were absorption obtained systems) (using spanning three upto clusters inferred from stellarthe internal evolution consistency arguments of this [25]. result,age e.g. for Although the this concerns observed star Th/Eu remainplagued [24], ratio implies about direct estimates a radioactive much of smaller dating globulare.g. avoids cluster many the inferred of ages values the had using uncertainties to the that be “Main revised have downward Sequence by several Turn-off Gyr Method”, following the consistent with the (recently revised) age of 11 calibration of stellarwhich distances obtained determine by the parallaxFunction”, distance see Figure measurements and 4) have [25]. been the developedof and age Other 10.5 yield Gyr a simultaneously methods [26]. minimum age (e.g. To forepoch obtain globular the clusters of age using galaxy/star of the formation, the universe to we “Luminosity the must add above2.2 number. The Hubble constant The age thusrived obtained from must measurements be ofSpace consistent the Telescope with present the Hubble expansion expansion rate. age of This the year universe, the de- If true this istheory) a of most a exciting very result[14, weakly since coupled 17]; it ultralight such implies scalar atuning, field the field which existence further is would (in exacerbating not mediate 4-dim stable thethe a effective 21 under cosmological ‘fifth cm field renormalisation force’ line constant and of problem hydrogen would yield [18]. require a conflicting massive bound However fine- at studies of PrHEP hep2001 =0is inferred Λ constant. 0 Ω , H H Subir Sarkar =1 m is 0 for a flat universe. κ As seen in Figure 6, 1 . 0 t =1whichhas 30%) systematic uncertainty Λ and Ω ∼ Simultaneous determination of 0 , H level and favours an open universe =0 σ m using a non-parametric modelling of the [31]. where the curvature term 1 Figure 4: the distance modulus andular age cluster for M55 the bytion” glob- the method (from “Luminosity Ref.[26]). Func- 1 κ − , this conclusion cannot as yet be considered − − 0 1, which equals 0.5 for the E-deS model where –5– H Mpc − =1 Mpc t 1 Λ 1 d − − / 1 +Ω with prsently a large ( − m 1 H − d 6kms Uintheold 11 km s ≡ , i.e. Ω ± Mpc ± 4 238 q 1 − eV) 3 =61 − 0 10 are the energy densities of matter and the vacuum in units of the critical density H × Λ 60 km s (3 Detection of ' ∼ 3 or a flat universe with a cosmological constant. However in view of the . N 0 0 and Ω H πG m . 8 , and -1 for a DeSitter (deS) model with Ω / 3 2 / 0 m 2 H t 3 It is often stated that an Einstein-DeSitter (E-deS) universe with Ω Here Ω ∝ 1 Figure 3: halo star CS31802-001. Synthetic spectrathree for assumed values ofcompared with the the abundance data are (from Ref.[23]). ≡ c too short-lived to be compatible with measurements of from the same dataset to 63 adopting the Keyage Project for measurement the universe of does the rule out present this expansion model rate at the and 2 a reasonable with Ω lenses [28], while similartainties) low in values three are other found systemsa (with [29]. value larger of Measurements possible of the lens SZE modelling in uncer- 14 clusters also indicate definitive. 2.3 The deceleration parameter The most exciting observational developments havethe undoubtedly deceleration been parameter in measurements of H [30]. Rowan-Robinson has arguedpeculiar that the motions Key using Project afor data more need metallicity sophisticated to effects be flow on corrected model for the than local Cepheid was actually calibration used, — and this also lowers the value of continuing debate concerning the value of lensed systems, and thewhich Sunyaev-Zeldovich effect bypass (SZE) the in X-rayobjects traditional emitting used ‘distance galaxy by clusters, ladder’ thetime and Key delays Project. or probe are far At beingB1608+656) present monitored deeper yield ten — distances the multiply-imaged two quasars than best have constrained the measured systems (PG1115+080 and International Europhysics Conference on HEP ρ PrHEP hep2001 Subir Sarkar yields the ‘luminosity (SCP) [32] and the 5 mag (correspond- . M 0 ∼ m The time-scale problem for an 10% uncertainties for the indicated is sensitive to the expansion history [34]. ) ± 0 0 z z ( d H z 0 R ) Figure 6: E-deS universe (markedsuming with a circle),values of as- the present expansion(from rate Ref.[27]). and age z 10 Gyr) are upto ∆ 20%, hence particularly well suited for cosmo- –6– ∼ . Supernova Cosmology Project =(1+ L d of a source of known absolute magnitude m ) + 25, where (HZT) [33]. Their basic observation is that distant supernovae L d Mpc 60%) fainter than would be expected for a decelerating universe such ' since then, thus the observed SNe Ia are actually further away than ex- 1 =5log( − Hubble diagram for Cepheid- 5 . M 2 − 1 corresponding to looking back m/ The obvious possibility that the SNe Ia appear fainter because of absorption by m ∆ ∼ 2 z speeding up Briefly, SNe Ia are observationally known to be a rather homogeneous class of objects, The measured apparent magnitude 2 Figure 5: calibrated secondary distance indicators; the bottom panel illustrates the decreasetuations in (due fluc- to peculiarcreasing velocities) distance with (from in- Ref.[27]). as the E-deS model.been This has been interpreted as implying that the expansion rate has pected. intervening dust can be constrained sincehas this would unusual also properties lead [35]). to reddeningthat It (unless the is the dust more distant difficult SNeof to Ia rule these out are possibilities the intrinsically by possibilityreview fainter. of the of evolution, the observing data Many i.e. teams and carefulthe their themselves implications analyses summary as for of have cosmology well the has been as been present made given by situation by others. below Leibundgut [37]; draws A largelywith detailed on intrinsic this peak work. luminosity variations logical tests which require aof ‘standard hydrogen candle’ in their [38].sion spectra They of [39] are a and characterised white are by[40]. the dwarf, believed absence However although to it result there is from is known the (using as thermonuclear nearby yet objects explo- no with “standard independently model” known for distances) the that progenitor(s) This has been donesupernovae (SNe through Ia) impressive carried deep out studies by the of the Hubble diagram of Type Ia International Europhysics Conference on HEP distance’: High-z SN Search Team (upto ing to 10 PrHEP hep2001 PrHEP hep2001 template It should be 15 Subir Sarkar m 06 magnitudes . 0 ± 20 . redshift z 06 using the MLCS method . 0 ± The observed colours of low red- .01 .1 1 14 . 0 .5

. It is therefore a matter for concern

1.1 (B-V) Figure 8: shift [42] vs highRef.[37]). redshift [33] SNe Ia (from –7– 04 for the SCP sample [32]. Thus the claim that . 0 ± 06 . (Phillips et al. 1999) B m (Phillips et al. 1999) A ∆ Riess et al. (1998) Perlmutter et al. (1997) .0.2.4.6.8 -.20 0123 correction method; this decreases to 0 5 than in an (empty) universe expanding at constant rate when analysed . 0 3 2 1 0 .4 .2 -.2 -.4 -.6

0 Comparison of the magnitude

B (other source) (other A m (other source) (other m ∆ m ∼ z Figure 9 shows the magnitude-redshift diagram of SNe Ia obtained by the two teams. corrections (top panel)absorption and (bottom of the panel)SNe inferred for Ia sample, the forods the nearby used three to different analyse meth- the data (from Ref.[41]). Figure 7: [42] or the Multi-colour Lightsimple Curve ‘stretch Shape factor’ to (MLCS) normalise usedemphasised the by that observe the apparent such HZT peak corrections [33]; magnitudes areso the [32]. essential SCP as to uses to reduce a the allow scatter in significant the cosmological data deductions sufficiently that, as seen incompare Figure well 7, with the eachproperty different other methods of [36, SNe make 37] magnitude Ia —Figure corrections was as 8, that responsible they distant do SNe should forthe Ia not if the derived appear indeed reddening observed to may some correlations. have bealso intrinsic been bluer finds physical Moreover underestimated in that [37]. as colur both A shownthat than teams recent in the nearby reanalysis have peak of ones; underestimated the luminosity-decay this thebefore data time suggests effects maximum correlation that of light is host are much galaxyacceleration weakened excluded; extinction [43]. if this and SNe further Ia weakens not the observed significance of the claimed Averaging over the distant supernovae, it is found that they are 0 fainter at using the ∆ International Europhysics Conference on HEP the time evolution of SNeintrinsically Ia brighter is ones tightly fade correlated faster withscatter — their in this peak the can luminosities Hubble be such used that diagram the to using make various corrections empirical to methods, reduce viz. the the ∆ as done by HZT [33],cosmic and acceleration to only has 0 beenfor the observed Higgs is at no LEP! more The significant best than fit the ‘ΛCDM recent model’ indications — a low matter density, cosmological PrHEP hep2001 Subir Sarkar ) so the expansion should 3 ) z 7—ishoweverfavouredby . 0 (1 + ' ∝ = 0 (long-dashed line), a DeS universe Λ = 7 universe (dotted line), with all data 1 the cosmological constant becomes Λ Ω Λ Ω , & 3 Ω , . z , 0 3 . =1 ' m =0 –8– m m 7 is claimed to have largely eliminated alternative . 1 ∼ z 1. It is argued [44] that SN1997ff is indeed a SNe Ia because − 3 / 1 ) method [42]. The lowest panel shows separately the Supernova Cosmology m 15 Ω / m Λ = 1 (dashed line), and a flat Ω =(2Ω The Hubble diagram of SNe Ia (top panel) compared with the expectation for an Λ z Ω , More recently the serendipitous discovery of a very distant supernova (SN1997ff) in the =0 m be seen to be slowingoccurs down at at such epochs; the transition from acceleration to deceleration measurements of theindependent age measurements and of the present matterhas expansion density become and the rate spatial new curvature as “standard to cosmological discussed be model” discussed, earlier, for so the and astronomical also community. by Hubble Deep Field atexplanations a redshift such ascosmological absorption constant by [44]. “grey” This dust is or because luminosity for evolution in favour of a unimportant relative to the increasing matter density ( Figure 9: empty universe (full line), an E-deS universe Ω International Europhysics Conference on HEP constant dominated flat universe with Ω Project data [32], and the middle panel, the High-z SN Search Team data [33] (from Ref.[37]). Ω normalised to the ∆ PrHEP hep2001 7 . 1 ∼ inde- z 3[47,48]. Supernova . 0 Subir Sarkar

∼

7 suggested by Reddening . 0 ,withanenergy m m ∼ Λ 1.0 SN 1997ff than would be expected for 2000 SNe Ia out to a redshift ∼ 1 and the data for SN1997ﬀ at brighter . is rather weak as argued above, many Λ z –9– determinations to a few per cent! Only through Λ 0.1 =0.65 =0.0 =0) Λ Λ =0.0 and Ω Λ Ω Ω Ω Ω m =0.35, =0.35, =1.0, M M M Coasting ( Grey Dust or Evolution Ω Ω Ω 7. This is indeed consistent with the value of Ω . 0 (SNAP) space mission [46] will observe ∼ 0.5 0.0

-0.5 -1.0

(m-M) (mag) (m-M) ∆ Ω − Hubble diagram of SNe Ia normalised to an empty uniformly expanding universe with 2 and the increase in statistics and improved control of systematics should be able observations which also suggest, that there is a substantial cosmological constant. . 1 0 i.e. the universe is spatially flat [50]. The second is that, as recognised for some ' z ' (from Ref.[44]. By the ‘cosmic sumclustered rule’ on this the then largest requires spatial that scales there probed be in some the form measurements of ‘dark of Ω energy’, un- density of 1 time, several types ofin observations gravitational suggest clustering that is the significantly amount less of than matter the which critical participates density, Ω pendent The first, as discusseddegree-scale angular by fluctuations Ganga in atzon the this (a CMB ‘standard conference provide rod’) [49], aκ at is measurement last of that scattering the recent [51] sound and measurements thereby hori- of indicate that the curvature term astronomers have nevertheless accepted the findings. This is really because of two such future measurements canenergy, the be claim definitively of tested. recent cosmic acceleration, driven by vacuum 3. The spatial curvature and theEven matter though density the direct evidence for a non-zero Ω Figure 10: redshift binned data from the SCP and HZT for redshifts International Europhysics Conference on HEP the host galaxy is anthe elliptical observed rather than colours a andthen spiral and time does this evolution. indicate identification is deceleration consistent As with in seen that in SN1997ff Figure is 10, the inferred luminosity an uniform rate ofluminosity evolution expansion; as being this responsible argues forredshifts. the against However curvature it absorption in has the by been Hubbleby dust diagram noted gravitational at that or lower lensing SN1997ff simple may dueline forms have of been to of sight significantly two [45] brightened foreground soit this elliptical does is galaxies not illustrate which quiteAcceleration the the are Probe clean effectiveness close ‘smoking of to gun’ signature such the hoped deep for. observations. However The proposed of to improve the accuracy of Ω PrHEP hep2001 , p/ρ =1. ≡ n w eV, much 33 − Subir Sarkar 10 ∼ ) 0 that the primordial H ( 1 MeV, nevertheless O ∼ assuming 1 [54, 55] i.e. a cosmological constant! neutrinos decouple at '− eV, thus raising formidable difficulties in relating it to 3 w –10– − after 10 × 3 ∼ 9 for the spectrum allows a high matter density universe to be consistent . 0 ' n However it should be emphasised that this is rather indirect evidence for a cosmo- 3 For example the position of the first acoustic peak in the angular power spectrum of Another important property of the primordial scalar density perturbation about which In seeking to understand why the vaccum energy has begun to dominate at the present epoch many 3 smaller than the height of its potential, fundamental theory. However if thethe dark SNe energy is Ia parameterised observations in are terms in of fact its best equation of fitted state by theorists have pursued the idea thatIn it order may to be be the affected energy by of an the evolving Hubble scalar expansion field such — a ‘quintessence’ field [52, must 53]. have a mass of the CMB implies thatdensity the perturbation, spatial presumably curvature generated isand during close gaussian. inflation, to This is zero is assumed onlythat indeed to if what we be the is have expected adiabatic primordial noperturbations, in scalar “standard simple e.g. inflationary model” including models ofbation isocurvature [56] in inflation modes. but an it given universe The filled ishas most with legitimate in photons, general fact to baryons, cosmological several neutrinos, such consider pertur-moreover and modes the more cold including adiabatic dark two general and possible matter isocurvature ones (CDM) ist modes in then are the the in neutrino inference general component from correlated. [57]; compromised the If position [61]. such of modes the ex- first Evenother peak if cosmological that parameters the we universe can assume ispresent be flat that [58]. significantly is the severely altered For if universedensity example, there inferred is from as are the flat, isocurvature shown CMB modes the data inthe with perturbation Figures extracted the is value 11 values adiabatic. indicated and by of e.g. Of BBN 12, course neutrino [59, isocurvature it 60] the perturbations holds is agreement only not if of clear what the physical baryon mechanism can excite logical constant and there maymatter be density other and explanations the for criticalthe the density conclusion, apparent required shorfall each by between line the the of CMB. evidence In ought view to of be the critically importance judged of on its own merits. International Europhysics Conference on HEP the SNe Ia dataenergy. leading to the widespread identification of the dark energy with vacuum with the data [63].in models Moreover of ‘new such inflation’the a [56] required departure and explicit tilt from modelsthe have scale-invariance based baryon been is on density supergravity indeed presented inferred which expected require do [64]. from yield such the a It recent spectral CMB is tilt data [49, interesting with 50]. to its note BBN value that [59] consistency now of does Indeed many of the argumentsthe for LSS a power low spectrum) matter aresmall density crucially ‘tilt’ universe dependent (e.g. on the this ‘shape assumption; factor’ even of allowing e.g. a this illustrates that “precision”cannot determinations yet of be cosmological considered parameters fullyand from robust. adiabatic the perturbations To CMB distinguish requires experimentallypossible measurements between with of isocurvature the the CMB forthcoming PLANCK polarisation surveyor which [61, will 62]. be we have no direct knowledgecosmological is parameters from its analyses spectral of shape. LSSperturbation and has It CMB a data, has scale-invariant been ‘Harrison-Zeldovich’ form common with practice power-law to index deduce PrHEP hep2001 4 Subir Sarkar = 1 universe no longer gives a m of a scale-free (and gaussian) The same but now for an as- (2dF) galaxy redshift survey [68] forest” in QAS [70]. It has also become α assumption Figure 12: sumed baryon density twicethe the BBN adiabatic value; case ismixed now case a (with poor 69%nent) fit is isocurvature but quite compo- the acceptable (from Ref.[58]). –11– Two-degree Field 02 indicated by BBN. . way to constrain such possibilities is through more detailed analyses =0 2 h b Fits to the CMB data assuming =Ω b ω independent In fact the primordial perturbation may not even be scale-free; indeed it is sensitive There has indeed been impressive recent progress in mapping the present day distri- The CMB data used (from the Boomerang experiment) has subsequently been revised but our conclusion 4 Figure 11: purely adiabatic perturbations (dashedand line) mixed perturbations with 12%ture isocurva- content (solid line), adopting thedensity baryon good fit asmight before. be advisable In to view await of data the from the uncertainties ongoing in MAP the mission measurements, [67] before particularly coming at to high a multipoles, firm conclusion. it still holds (e.g. using the latest combined data set [50]), although an Ω to the dynamics of inflationis (and likely the to behaviour of haveprediction fields features in other such this than as the regardshould inflaton ‘steps’ so still itself) and it and assume ‘bumps’ is athe quite [65]. scale-free data). appropriate Of spectrum that However course analyses (butallow the of there now for inferred CMB is allowing a cosmological and no a morehigh parameters LSS unique ‘tilt’ matter complex can data density for primordial change universe consistency spectrum, to substantially with be if e.g. consistent we with a the ‘step’ LSS and in CMB the power spectrum spectra [66]. permits a primordial perturbation. bution of galaxies [68, 69]higher and redshifts in through probing studies of thepossible early the to “Lyman- ‘linear’ probe epoch of theduced structure by dark gravitational formation lensing matter at [71]. distribution The directly through studies of the ‘shear’ in- The only of structure formation, to test the important International Europhysics Conference on HEP has detected the expectedvelocities distortion induced in by the redshift-spacedark gravitational due matter collapse to distribution, of peculiar asthe structure (i.e. is [68]. same indicated non-Hubble) data, by If studies galaxies then of do this high-order trace does correlation the functions provide in direct evidence for a low density universe with PrHEP hep2001 7which . 0 ∼ Subir Sarkar Λ .Thesituation Ω , 3 . 0 ∼ m 85, [72]. With improving datasets . 0 explanation in terms of fundamental < Λ no Ω < However we are certainly not without doubt! 65 . –12– even wrong” “Cosmologists are often wrong, but never in doubt” “...not 06 on the largest scales (upto several hundred Mpc) probed so far. This is . 0 ± 27 . This is of course not trivial — general relativity has been extensively tested on (rela- Landu famously said =0 m is consistent with all astronomical data but has today is perhaps better capturedof by the Pauli’s enigmatic data remark — may the be present interpretation physics. One might hope tocontent eventually find of explanations the for the universeappears dark matter in to (and the baryonic) be context littleuniverse of prospect — physics of the beyond doing cosmologicalscience the so so constant. it Standard has for been Model the appropriate Cosmologyin to but apparently has the consider there only dominant framework in the component simplest of theconfrontation possible of general cosmological between past the models relativity. particle been physics However apossible and now alternatives data-starved cosmology, that it to we is general perhaps are relativity. time faced to with even thistively consider small) serious astronomical scales [73] anda the successful standard account cosmology of based on observationsthat it back the certainly to ferment gives the of BBN currentmodifications to era theoretical our [60]. ideas notions about concerning However gravity, evenwhich ‘brane-world’ it sacred might turn is texts like suggest not out the small unlikely Equivalence to Principle, are be entitled to, significant and in willBut the continue it to, cosmological is analyse important context their for data [73]. itparameter in to among terms be Of many of recognised course well-established specifying that astronomers physics. the a cosmological cosmological constant model. is not just another Moreover the crisis posed bycosmology the recent alone; astronomical it observations is iscessful the not creations one spectre in that that physics confronts cosmology haunts — is any quantum inextricably attempt field intertwined with theory to thatmost and unite of exciting general fundamental two physics indeed. relativity. of and promises the The to future most be of suc- it should soon be‘priors’, possible in to particular test the natureconcerns the of about robustness the structure of primordial formation such density [47]. results perturbation, with and even address fewer other assumed also consistent with theCMB measured data (assuming power a spectrum. scale-freeis found primordial By for spectrum doing a of a cosmological adiabatic joint constant fluctuations), with fit evidence 0 to the 2DF and Acknowledgments I am grateful tospondance Bruno concerning Leibundgut and the Michael measurementthank Rowan-Robinson of the for organisers cosmological very of parameters. helpful this corre- excellent I conference would for also the like invitation to to present this review. 4. 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