As explained in this earlier post, for this week and the next instead of running my usual weekly arXiv column, I’ll use this blog as a conference diary for two conferences I’ll be attending. I don’t yet know how this is going to pan out (in terms of format, etc.), I guess I’ll figure out on the fly what’s the best way of doing this, but don’t expect anything neat and orderly . This is pretty much the digital equivalent of the type of things I would jot down on the conference notepad if these conferences were happening in person (including possibly some extra-physics stuff from time to time, depending on how bored I am?). As such, take whatever is written here with a grain of salt. I hope you enjoy it!
TMCC DAY 1
Adam riess (jhu/stsci) - NEW DETERMINATION OF THE HUBBLE CONSTANT WITH GAIA EDR3, FURTHER EVIDENCE OF EXCESS EXPANSION
Tehran looks a lot like Adam’s office (ipse dixit)
Local distance ladder doesn’t assume GR or LCDM but I always thought the deceleration parameter was fixed to q0=-0.55 i.e. standard LCDM with Om=0.3? Hmm
Uses GAIA EDR3+2020 HST sample. The H0 tension is equivalent to a 0.2 mag offset of local vs inverse distance ladder (makes sense, and for me this is why you shouldn’t use a local H0 prior but rather local M_B prior?)
FAQs: giant void (no), HST WFC3-IR flux scale linear to 1% (yes), Cepheid crowding (no), difference between SNeIa at ends of distance ladder (no)
Adam says best solutions to tension are strong neutrino interactions, early dark energy, evolving electron mass, early recombination, PMF. Don’t need a Nobel Prize to disagree :)
Quotes EDE as being “most likely” from Hubble Hunter’s Guide but really it’s “least unlikely”
Don’t sweep “problems” under the rug (precession of Mercury, solar neutrino problem, missing baryon problem - is this really solved?), is the CMB cold spot really a problem?
Look up for Di Valentino et al. 2021 upcoming H0 tension review (Di Valentino, Mena, Pan, Visinelli, Yang, Melchiorri, Mota, Riess, Silk?)
Adam sees potential for cosmic chronometers to shed light on the tension, but there is model-dependency there in the sense of complicated astrophysics. Systematic uncertainties not quantified in the model space (but isn’t this what Moresco et al. 2020 did? So I sort of disagree)
george efstathiou (cambridge) - The planck view of LCDM
George claims no inconsistencies in Planck, no tension between temperature and polarization, if your experiment disagrees with Planck either you are wrong or there is new physics beyond LCDM
Paris vs Cambridge likelihood (Plik vs CamSpec) gives A_lens != 1 at 2.8 vs 2.1 sigma, difference is in polarization and in particular treatment of polarization efficiencies
If we are looking at a fluke and I improve the S/N, then the anomaly will get “better”. My thought: this is true in an ensemble sense, i.e. it’s “most likely” it will go down, but for one particular anomaly we are looking at, we can’t be sure if it went down because it’s truly a fluke or because of a fluke-in-a-non-fluke. No way to know which of the two is true. Argument is that A_lens and correspondingly Omega_K anomalies are flukes because their significance goes down as S/N (and sky fraction) go up
Quieter residuals with new CamSpec likelihood visible by eye. And certainly true that all anomalies got “better” with more sky. So no hints or patterns for new physics have emerged, unlike in the past
One has to be able to justify a flat prior on Omega_K which affects the results. My thought: true only insofar as we believe in inflation, for which Omega_K=0 is an attractor
ACT DR4 fares well against Planck - so ACT has George’s blessing :)
Arman Shafieloo: even with your new CamSpec likelihood, you still get ~2 sigma anomalies (A_lens, Omega_K), aren’t these worth investigating more? I guess this will depend on whether you are comfortable with labelling 2 sigma fluctuations as flukes, and testing whether this is indeed the case by extending the multipole range (e.g. with ACT DR4). But I like George’s example of WMAP vs Planck to explain why we should expect fluctuations (though I can’t 100% agree that these are definitely fluctuations - see fluke vs fluke-in-a-non-fluke above). However, I still think one should be careful not to dismiss this, see the earlier don’t sweep “problems” under the rug by Adam
Adam Riess: Planck+external usually pulls up H0 from 67 to 68-69, asymmetrical around Planck’s central value, why? Maybe this is just a numerological curiosity, but interesting point. Or it could be a small fluctuation bringing Planck’s H0 slightly down from the truth
Overall thoughts: fantastic talk (as usual for George), very intriguing, but I’m not convinced that these anomalies decreasing in significance with higher S/N definitely proves that they aren’t anything more than anomalies (to the extent that we can prove anything in cosmology)
wendy freedman (chicago) - Local Measurements of H0: Is There a Crisis in Cosmology?
TRGB vs Cepheids. TRGB method based on equivalent of HR diagram. Number of systematics smaller than Cepheids or at least more manaageable
If we want to claim new physics need accuracy to ~1%
Advantages of Cepheids: long history of use and testing, luminous, small dispersion in Leavitt Law, can empirically test for systematics
Cepheids challenges: can we get precise enough photometry to measure H0 to ~1%? Clearly want an independent test of the Cepheid distance scale -> great motivation for TRGB!
Adam in chat: “Oh that’s the worst example!” -> associated to the previous slide (i.e. above line)?
TRGB advantages: simplicity, minimal corrections needed, crowding and blending not an issue, minimal effects of dust and metallicity, can be applied to both spiral and elliptical galaxies, local and distant galaxy types are similar
TRGB challenges: fainter stars (in practice not an issue, you don’t measure periods), dust in calibrator galaxies (previously largest sys!), small number of calibrators
There’s some inconsistency in the Cepheid calibration (see George’s lockdown paper)
Adam says that the fidelity on Cepheid photometry is more optimistic than depicted by Wendy, and there is also less distinction re crowding and blending
Interesting debate between Adam and Wendy cut short
BEN WANDELT (IAP/CCA) - Towards solving the cosmological inference problem
Cosmological inference problem: how do we know that we have chosen the most informative summary statistic? Are our approximations (statistical modelling, physical modelling) adequate?
(good question I always wondered this - can you find a [non]-linear combination of summary statistics which is the most efficient one? What does efficient mean? Maybe Ben will answer this)
BORG: model entire dataset, i.e. full galaxy map, ab initio (but what’s the likelihood? Gaussian voxel per voxel?) I worry this sweeps a lot of dust under the rug (do we really understand galaxy formation that well that we can do this?) - basically sample parameters+initial conditions?
Why is BORG not the end? Need more reality, better ability to mask data against model error, internal robustness against model mis-specification. But can you write a likelihood which includes everything??? This is where ML comes in - neurophysical engines respect basic physical symmetries (but I thought e.g. neural networks could learn rotational/translational invariance and locality even if you don’t teach it to them e.g. recent work by Shirley Ho?)
Interesting way to derive generalization of Alaock-Paczynski test+multi-tracer global Alcock-Paczynski test as an “optimal” cosmological test
The answer seems to be IMNN: information maximising neural networks, they extract the most informative summaries from the data
Behnam Javanmardi - Inspecting The Cosmic Distance Ladder
Inspecting the second rung of the Cepheid distance ladder, i.e. distance to SNeIa host galaxies -> is this the most promising rung for systematics which might solve the H0 tension?
Upcoming paper: “Inspecting the Cepheid distance ladder: The HST distance to NGC 5584”, done using SH0ES data
Why NGC 5584? Revisit amplitude-ratio relations
No systematic hints towards a solution to the H0 tension from systematics in the second rung, results completely independent from SHE0S (they weren’t even aware of this work)
Adam and Wendy both very happy to hear about this, effusive thanks for Benham
NICK KAISER (ENS) - Gravitational Lensing and Cosmological Parameter Estimation
(missed this unfortunately)
TMCC DAY 2
SUBIR SARKAR (OXFORD) - TESTING THE COSMOLOGICAL PRINCIPLE
Are we really still using the Friedmann equations despite all the data we have? Challenge homogeneity and isotropy now that we have the data. Weinberg has interesting admonition on this
CMB dipole due to pull from local inhomogeneities, relative velocity wrt frame where CMB is isotropic. But on what scale is this pull from? Great Attractor never found, moving towards Shapley
Are velocities falling down as 1/r as we move towards Shapley? No, seems constant (2011 work with Arman Shafieloo on Union2.1 SNeIa), so can’t be end of story, need pull from ~10^17 M_Sun supercluster? How unlikely is that, <1% of observers should experience a bulk flow of this magnitude -> WE ARE NOT TYPICAL OBSERVERS (failure of cosmological principle???) -> usual covariances inapplicable?
At 300 Mpc peculiar velocity variance should have dropped to 100 km/s (standard model calculation robust to shape of matter power spectrum), but this doesn’t happen in real data, bulk flow much larger than expected and continuing much deeper than expected
If kinematic interpretation of CMB dipole correct, should see similar dipole when looking at different sources: flux-limited catalog, see more sources in direction of motion -> do on real data, with NVSS+SUMSS, WISE, and quasars
Use quasars, can neglect clustering dipole -> our peculiar velocity wrt quasars != our peculiary velocity wrt CMB, direction correct but magnitude too high (rejected CMB dipole is kinematic at 4.9 sigma)
Why is this important? SNeIa analysis translate heliocentric to CMB frame, but translation stops at 150 Mpc (frames have converged) -> what we see on the sky is the dipole acceleration, much larger than monopole, not due to Lambda, are we preferentially looking at region of sky which favors Lambda interpretation?
Take-away message: acceleration inferred from SNeIa anisotropic, cannot be due to cosmological constant which would be isotropic
Corollary: H0 tension not well-posed issue, notion of H0 is itself ill-defined
PAVEL KROUPA (BONN/CHARLES UNIVERSITY, PRAGUE) - FROM BEAUTY TO REALISM: THE OBSERVED UNIVERSE IS NOT LAMBDA DARK MATTER
Standard model of cosmology (SMoC) makes statements about how galaxies should look like, gives rise to many disfunctional properties (see below)
Cannot predict how rotation curves of disk-like galaxies should look like? (missed one property) Too many compact groups of galaxies exist. No common infall solution for MW satellites. Plane of satellites. Local Group “too” symmetric! Void in local volume too empty. Density contrast vs cosmological distance. (missed another property) Too many merging clusters at high-z (Bullet Cluster and El Gordo).
Dysfunctional properties imply non-existence of DM halos or of DM (claim: scientifically suspicious in the same way you can’t claim non-existence of Higgs)?!? DM is a complete failure. I guess I don’t agree because doesn’t balance evidence pro vs against DM
MOG falsified >5 sigma, gravitational dipoles >>5 sigma, scale-invariant GR 200 sigma, emergent gravity authors too kind to put a sigma (LOL!!!). Don’t make people think Bonn = MOND, they take MOND on equal footing, it simply does incredibly well (would like to hear the cosmology side). Didn’t know that MOND was related to quark confinement, as empirical as Newtonian gravity, stop saying MOND is phenomenological
MOND symmetry: space-time scale invariance. Works until ~Gpc scales, looking at galaxy formation, doing first ever cosmological simulations
EVA-MARIA MUELLER (OXFORD) - THE COSMOLOGICAL IMPLICATIONS FROM THE BARYON OSCILLATION SPECTROSCOPIC SURVEY
As far as cosmology is concerned eBOSS Is the end of SDSS
Optical fibers had to be plugged by hand (by some poor UG student?)
Angles on the sky -> distances; changes in redshift -> expansion rate (of course all relative to the scale you are measuring i.e. sound horizon at baryon drag)
Get f*sigma8 measurements from peculiar velocities, always wondered at what effective scale these are measured (important if you want to constrain extended models with scale-dependent growth, e.g. modified gravity, massive neutrinos, pretty much anything other than LCDM+GR)
2 sigma tension with Ly-alpha data completely went away!!! (essentially due to increase in sample and increased S/N, reminded me of George’s talk yesterday about flukes going away -> this is a good example of what George meant)
QSO RSD results a bit off Planck (too high), ~1.5 sigma fluctuation, perfectly compatible with fluke I guess
Detection of DE from eBOSS alone (of course with all caveats about underlying homogeneity and isotropy assumption etc. etc. etc. see talks by Subir and Pavel)
Universe is flat (BUT Planck T+P in strong tension with BAO! So one should be careful about this)
Really need CMB+BAO+SNeIa if want to constrain very general models (e.g. time-varying DE+curvature)
Constraints on neutrino masses great but inclusion of RSD data subject to caveat I wrote above on f*sigma8 and the scale at which it is quoted (in my opinion better to use FS data than compressed RSD, I think other papers advocated this too)
SDSS can’t arbitrate S8 tension
Very nice talk, look out for DESI, Euclid, NGRST etc.!
MARIKA ASGARI (ROE, EDINBURGH) - KIDS-1000: COSMOLOGY FROM THE KILO DEGREE SURVEY
Good question from Marika to Eva: did you do any internal consistency test (as in KiDS)? Checked consistency of Fourier vs configuration space, good agreement, although I’m not sure it’s exactly the type of consistency test Marika meant
Look for coherent distortions across galaxy shapes
Unblinding party with poll for which distribution for Omega_m-sigma_8 they would have preferred (out of 3 distributions), majority preferred strong tension with Planck (reflects the young average age of scientists in KiDS, excited by tensions -> name KiDS is appropriate :) ), in the end the real distribution turned out to the the intermediate one -> importance of blinding data analysis!!!
Removing highest two redshift bins (bin 4 and bin 5) has quite large effects on results (lowest redshift bins have low S/N, so unsurprising)
Consistency test showed bin 4 and bin 5 consistent with each other, surprisingly the “strange” bin is bin 2, but keeping or removing it doesn’t change anything (still trying to understand what this means or what are its implications)
Combining KiDS+BOSS breaks cosmic banana!!!
No obvious extension can solve the tension with Planck
ALEXANDRA AMON (STANFORD) - THE STATUS OF THE DARK ENERGY SURVEY YEAR 3 COSMOLOGICAL ANALYSIS
Check out the #DESendofnights hashtag on Twitter
3*2pt helps to constrain nuisance parameters -> maximizes cosmological information (basically you can internally calibrate nuisance parameters such as biases etc.)
Tense looks on unblinding telecon, how many sigmas tension?! :) (SB [normally I would write the name on my notepad, but this is public] looks particularly concerned!)
It’s not just throwing more data at the Y1 pipeline, so even without cosmological parameters (as she was not allowed to show them yet) a Y3 talk is very interesting! Huge improvements in analysis method, particularly with regards to systematics
Detection of B-modes would be a good indication of systematics - but I’m wondering is there some new physics which would give a genuine B-mode signal in WL? Reminds me a bit of how <EB>=0 is used to calibrate the CMB polarization angle
Consistency of 3 methods was unblinding requirement
Whether or not photometric or spectroscopic samples are used doesn’t affect cosmological constraints! (interesting plot even with blinded axes) - constraints stable against redshift calibration
One of the key systematics is blending leading to z-mixing effect
Even at relatively shallow depth of DES shear calibration is important (else bias to low S8!!!)
Results consistent across z-bins, angular scales, and statistics
Nice laser-cut jigsaw puzzle
Y3 S8 results are starting to rival Planck precision
Lots to look forward to in DES Y3 - delayed because of elections, Thanksgiving, Christmas (DES gave one month off), pandemic!!! Marika Asgari: when should we see the papers? We are more or less at ~t-1 month
DICK BOND (TORONTO) - COSMIC POWER FROM CMB AND LSS SIGMA_8 FLUCTUATION PROBES
Webskying mini-industry at CITA (basically correlated maps of every possible interesting cosmological signal from every possible cosmological probe you can think of)
What is the characterization of power beyond sigma_8? Primordial power spectrum of zeta or whatever gauge invariant variable you choose to describe curvature fluctuations in the early Universe
Made me think of interesting talks by Ariel Sanchez about using sigma_12 instead of sigma_8
Old measures of power: P_z, J_3, b_8, and finally sigma_8 -> 8 h^-1*Mpc associated to the size of the Abell Cluster
sigma_8 is usually quite stable against extensions to LCDM (opposite of curvature and H0), one exception are massive neutrinos (and I guess stuff like decaying DM, Dick mentions fuzzy DM)
Sources of tension: sigma_8 from clusters, some tension with Websky Compton-y map
Hard to make a case for fundamental physics from the sigma_8 “tension” (I guess as of yet?)
TMCC DAY 3
JOE SILK (OXFORD/IAP/JHU) - Are dwarf galaxies a challenge to LCDM?
Missing satellites problem (dwarf counts but also other aspects too, I guess too-big-too-fail is one of them - diversity, core vs cusp, radial deficiency, satellite planes)
MSP has gone away with introduction of right physics in modelling of dwarf galaxies (dynamical friction, tidal disruption, stripping by gas, and so on - small satellites are most easily perturbed, become so diffuse you can’t see them!!!)
Diversity problem: too much scatter in circular velocities, something missing in modelling of galaxies, but putting in baryonic physics seems to go in the right direction
Core-cusp problem solved by baryonic feedback (SN)+allowing for non-sphericity of motion of DM particles - no need for non-CDM? PBH could solve this as they heat galaxies and change cusps into cores (was used to rule out too many BHs), need few % of DM is PBH
Radial deficiency: combination of dynamical friction and tidal disruption solves all the problem
VPOS only 16% unlikely (satellite enhanced by LMC?)
TBTF solved by AGN feedback?
DM-deficient UDDG explained by high-velocity collisions of gas-rich dwarfs?
Excess of globular clusters in UDDG, this might mean we don’t understand them enough
CDM predicts wandering IMBHs in dwarfs
Are exotic DM models motivated by claimed problems with dwarfs? No according to Joe, threshold for new paradigm not reached once include baryonic physics, at most they provide hints (and certainly can’t be explained in DM-only simulations) -> interesting, I thought Joe’s talk was going to argue for dwarf galaxies being a challenge to LCDM, but I guess this is just one more confirmation of the rule that if a paper/talk has a question in the title, the answer is “No”
Take-away message: no need for new physics to explain dwarf galaxies, it’s all just baryons+BHs!
eleonora di valentino (durham) - investigating cosmic discordances
At least 3 approximations in LCDM are theoretically ad hoc: inflation, CDM, Lambda!
Main tensions discussed by Eleonora: H0, Alens, S8, Omega_K
To solve H0 while fitting CMB, R19, BAO, need something which reduces sound horizon, but these early-time modifications typically in tension with galaxy clustering data
IDE has high H0 from Planck alone, but has problems with BAO and SNeIa (lowers tension to 2.5-3 sigma, could be residual statistical fluctuation), most importantly from CMB alone you get a fake detection of IDE because of the geometrical degeneracy
Alens can explain internal tension in Planck
S8 tension is 3.4 sigma (but not obvious to me this is an early-late tension as H0)
Varying Alens changes S8 from Planck, solves S8 tension. Alens is consistency parameter, so not sure how to interpret a model where Alens is allowed to vary, proxy for new physics (modified gravity??)?
In 11-dimensional parameter space Planck alone consistent with R19, but again BAO and SNe are the problem
Closed Universe explains Alens (see George’s talk above for statistical fluctuation interpretation)
Very good point Eleonora only makes: within a certain model, you can only combine two datasets if they are consistent within the given model, i.e. they can plausibly arise from the same realization
Planck+Pantheon indicates Omega_K=0 only for 7 parameters, for 11 parameters Omega_K=0 and w=-1 ruled out, phantom closed model
LCDM ruled out?
Yacine Ali-Haïmoud (NYU) - cosmology from the CMB FREQUENCY SPECTRUM
Didn’t know that also COBRA measured CMB spectrum aside from COBE-FIRAS!
Guaranteed signals in vanilla LCDM
Fractional deviation from perfect blackbody ~integral of logarithmic derivative of energy injected for z<2*10^6 (has to be less than 10^-4 because of COBE-FIRAS limit)
Guaranteed signals: baryon adiabatic cooling, recombination radiation, dissipation of acoustic waves, SZ signal from hot intracluster gas
SDs from DM interactions not competitive with CMB anisotropies only for s-wave annihilation
T0 has similar effect to H0 on the CMB (proxy for age of Universe)
Essentially T0 subject to geometrical degeneracy, can be broken by BAO, which seem to be in favor of COBE-FIRAS. T0 doesn’t solve tension, but shall call it Hubble-Penzias-Wilson tension?
contributed talks (session wa)
LUKE HART (MANCHESTER) - reading between the lines: the future of cosmological recombination
Non-equilibrium processes in recombination can imprint signatures in CMB spectrum?
Essentially use SD to detect atomic transitions occurring during recombination
Interesting plot of logarithmic derivatives of intensity as a function of some interesting parameters
[MY TALK]
Didn’t take notes about this of course
ABDOLALI BANIHASHEMI (SBU) - Critically emergent dark energy as a proposal to address both spatial anomalies and H_0 tension
Is H0 a temporal or spatial tension? (temporal: early vs late tension)
Dark energy is the result of a phase transition? (Ginzburg-Landau theory)
DE = <phi>, phi order parameter <> spatial average, free energy respects Z_2 symmetry
Has high H0 (using CMB alone)
Dark energy patches can solve LCDM spatial anomalies? Planck small vs large ell, power asymmetry
COLIN HILL (COLUMBIA) - Exploring Cosmological Concordance with ACT DR4, Planck, and Beyond
ACT doesn’t get lowest multipoles in temperature (due to scales being too large?), so important to combine with other experiments e.g. Planck and WMAP -> subject to consistency!!!
Think of ACT as independent check of WMAP and Planck
ACT+WMAP has almost same statistical power as Planck
Biases on H0 from nonlinear CMB lensing - nonlinear evolution and baryonic effects on small scales in CMB lensing power spectrum. “Set it and forget it” for HALOFIT and HMCODE not enough for SO and S4, not a problem for Planck, ACT, SPT currently
Up to 1.6 sigma bias on H0 in CMB-S4 (lmax=5000)
Default accuracy setting in Boltzmann codes for lensing calculations will need to be improved for SO and S4 (not difficult, just increase the default accuracy), otherwise biases of several sigmas
EDE in trouble: no hints for EDE in Planck alone; error bars larger, but central value of H0 low; low H0 if don’t include R19 prior
Constraints driven by prior volume effects? No!
EDE doesn’t solve H0 tension
MIKE HUDSON (WATERLOO) - Cosmic flows crank up the tension in cosmology
(missed this unfortunately)
TMCC DAY 4
contributed talks (session T1B)
elena asencio (bonn) - El Gordo, a massive blow to LCDM cosmology
Very massive galaxy clusters moving at very high infall speed -> problem for LCDM hierarchical structure formation paradigm?
How rare is El Gordo? Search for analogs in cosmological simulations (pre-merger configuration)
Actually look at cumulative PDF, then rescale -> p = 7.51*10^-10 -> 7.5 sigma
Combine El Gordo+Bullet Cluster -> 6.43 sigma
Can explain with MOND gravity+ 11 eV sterile neutrino (but can this explain BAO? I guess Moritz will talk about this later)
Konstantinos Migkas (bonn) - Is the local Universe anisotropic? Galaxy clusters seem to think so
Galaxy clusters should have same properties in a statistical sense, no bulk flows
L_X vs T vs Y_SZ, use scaling relations -> cluster temperature is key quantity (cosmology-free!!!!!)
Need cosmology to calibrate scaling relations -> constrain H0, DL, DA in multiple sky directions
Is H0 isotropic? 4 sigma anisotropy (66 vs 73), multi-wavelength 5.9 sigma
Reasons: excess X-ray absorption, bulk flows, or local cosmological anisotropy
Moritz Haslbauer (bonn) - The KBC void and Hubble tension in LCDM and Milgromian dynamics
Local underdensity across whole electromagnetic spectrum: KBC void
LCDM expected rms density fluctuations 0.032 -> <delta> ~ 0.46±0.06 for KBC void
KBC excludes LCDM at 6.04 sigma (7.09 sigma with Hubble tension)
Cosmological MOND with 11 eV sterile neutrino (standard expansion history as LCDM)
But can this explain tracer power spectrum (I’m mostly thinking of Ly-a flux power spectrum)?
contributed talks (session T2B)
shao-jiang wang (CAS, beijing) - Could Hubble tension be solved by late-time new physics ?
Chameleon dark energy coupled to overdensities
Keyword is late-time *inhomogeneous* solutions! (homogeneous excluded by BAO data)
Need 20% overdensity, Great Wall overdensity?
Can’t self-accelerate, conformal coupling varying ~O(1), still need CC or quintessence effect
Can also solve S8 tension?
hossein mos'hafi (ipm, tehran) - CMB lensing and Hubble tension in üLCDM
Ensemble theory of gravity: density-dependent transition of laws of gravity
Gravity model works as ensemble average of all possible theoretical models
Uber-gravity uses all possible powers of Ricci scalar (but it is not renormalizable, right?)
(is xi a Lagrange multiplier? If so it should be varied? But that would tell you R=R0?)
Seems somewhat related to Parker vacuum metamorphosis
shouvik roy choudhury (iitb) - Strong Neutrino self-interactions and the Hubble tension
Flavour-universal neutrino self-interactions (heavy scalar or heavy vector)
Neutrinos keep interacting even after 1 MeV, they don’t free-stream until later
Need higher Neff, non-zero Mnu to compensate high Geff (>10^9 GF) otherwise too much power
As soon as high-ell polarization data added, exclude high Neff/high H0
Strongly interacting neutrinos don’t solve H0 tension and problematic for particle physics
glenn starkman (case western) - an uncooperative universe
(missed this unfortunately)
contributed talks (session T3B)
mohammadreza ayromlou (MPA) - No Place for the halo boundary: how environment influences galaxies up to large scales
Importance of removing halo boundary
Environmental effects happen beyond halo boundary -> have to eliminate artificial boundaries
Take-away message: when modelling galaxy evolution have to remove halo boundary!
Alex Hall (edinburgh) - The impact of our local environment on density and lensing power spectra
Does our dense local environment affect the LSS we see via spatial correlations in the density field?
Leads to % level effects in WL power spectrum across all scales -> important for Euclid if true!
Turns out to be negligible for upcoming surveys (unlike claims of Reischke et al. 19)
Thomas Kite (Manchester) - Bridging the gap: spectral distortions meet gravitational waves
GWs coupled to anisotropic stress in medium (e.g. neutrino free-streaming)
Anisotropic stress affects wavelengths which are relevant for SD
GWs source quadrupole -> heating rate -> SD
SD close gap in frequency space between CMB and other probes (over 7 decades in frequency)
Example: audible axion (my note: this also interesting for NANOGrav)
levon pogosian (sfu) - the hubble tension and the magnetic universe
Treat rd as an independent parameter, so don’t make assumptions about recombination, if you know omega_m can break rd-H0 degeneracy with BAO at different z - where can measurement of omega_m come from? CMB lensing, galaxy lensing, cosmic chronometers, or just a prior
Important consistency test especially with future data!
Proposed models: a) many models proposed with aim of solving the Hubble tension; b) primordial magnetic fields (provocative but I like this!)
Need to explain muG magnetic fields, were already there, compressed with a help from dynamo
Evidence of B-fields on Mpc scales from blazars?
Stochastic PMF, generated during phase transition, amplitude decreases as a^-2, scale-invariant PMF spectrum if generated during inflation
How do they solve Hubble tension? Induce baryon inhomogeneities on small scales (below photon mean-free-path), speeds up recombination, smaller sound horizon, basically because in the recombination equation <ne^2> > <ne>^2, derivative more negative, recombination proceeds faster
Planck alone no detection of clumping factor b (related to <ne^2>), degeneracy b-H0 (so I suspect this comes from H0 prior alone?), so get higher H0, can relieve S8 tension
Need 0.05-0.1 nG magnetic fields -> special value, so good to get excited
Can’t solve H0 tension consistently with BAO without increasing omega_m (keeping rd as an independent parameter, different from what done usually)
The above worsens S8 tension! So problem with WL and LSS
General argument: can’t solve H0 tension by lowering rd alone
lloyd knox (uc davis) - can additional light relics restore cosmic concordance?
Obviously as Lloyd himself says the answer is no since the title is a question :)
Answer could become yes if we can find ways to mitigate effects which break BAO scaling: photon diffusion and free-streaming
BAO scaling takes H(z) -> alpha*H(z), BAO observables unchanged -> essentially keeps angles unchanged, but need to boost all three types of densities, so need to increase radiation density, and need extra light relics. Does the Blinov & Marques-Tavares model work?
Photon diffusion breaks BAO scaling since r_d^2~1/H, this is why it’s hard to decrease sound horizon without messing up small-scale polarization -> increase Neff increase damping
Key point: n_e(z) sensitive to H(z) because recombination is clearly a non-equilibrium process
Need low Helium Y_P to get high H0
Caveats: something could impact recombination
SPT-3G will put even more pressure on light relics as a solution to the H0 tension
Take-away message: BAO scaling and recombination clock effect important, polarization severely constrains light relics even when more parameters (Helium, free-streaming fraction) introduced, recombination clock effect important for constraining early-time attempts to solve the H0 tension (it’s of course already included implicitly in the calculations, but isolating the effect may help to pin down where problems are), more to look forward to from SPT-3G
OVERALL THOUGHTS
This was a fantastic conference! The organizers did a great job, were super kind, and Persian music is lovely (excellent musical taste especially by Nima and Shant).
3 interesting things I learned:
there are many challenges to LCDM (is LCDM definitely off the table?) and possibly the cosmological principle not so much from cosmology but on smaller scales (clusters, galaxies), but dwarf galaxies are not one of them
difficult to solve H0 tension with early-time new physics alone, simply lowering the sound horizon not enough, have to keep an eye out on omega_m, recombination clock useful in this sense, definitely something to further explore, look out for work by Cyr-Racine, Ge, and Knox
my personal prediction: information maximizing neural networks will become a huge thing in the field in the future (and not even that far in the future)
extra-physics thoughts
Arthur injured, we need a playmaker in midfield, Rabiot, Bentancur, and Ramsey su*k in that role. Considering we have many central defenders, I would play either Bonucci or Danilo in midfield as they are good in setting up play and keeping the ball (though of course not as good as Arthur). Or Pirlo may play Fagioli. Looking forward to tonight’s Juve-Crotone. Surely we must win this one (either that or we have lost the scudetto)
Building sites are so interesting
Riess et al.’s results are the most “Riess-ent” ones (did anyone ever come up with this pun? Probably yes. Not proud of it)
Fantastic choice of musical background for the Day 2 breaks by Shant Baghram: Mahler’s Symphony 5’s slow movement (movement 4, adagietto) and Ständchen (Serenade) by Schubert, adapted for orchestra (?) - I’ve always thought Schubert is one of the most underestimated classical composers (by classical I mean loosely speaking baroque+classical+romantic+post-romantic), alongside the likes of Händel (and many others)
Would be really nice to visit Iran!