ESO has organized an important online conference (22-26 June 2020) on the current significant tension in measuring the expansion rate of the Universe, i.e. the Hubble constant H0. The video of each day’s talks and the panel discussion is uploaded on YouTube by the organizers and is publicly available.
Here is a summary of day four of the meeting with links to specific parts of each speaker’s talk mainly for future personal reference, but hopefully useful for others as well. Check previous summaries here: day 1, day 2, and day 3.
After Antoine Mérand announced the start of the second day, the talks went on as follows:
Hendrik Hildebrandt, Weak gravitational lensing
- Intro:
There is also a tension between weak lensing observations and Planck, which could provide insight on H0 tension.
Intro to cosmic shear: measuring the ellipticities of many galaxies and inferring about the matter distribution in the large scale structure (LSS). The light fro galaxies get continuously deflected and distorted by the structures along the way. This will cause a statistical change in the random orientation of galaxies producing an alignment.
Cosmic shear is sensitive to matter distribution and geometry.
Main observables: ellipticities of a large number of galaxies and their photometric redshift, photo-z. - Two-point shear correlation function:
Measures how strongly galaxy ellipticities are aligned as a function of their relative distances. It is related to the matter power-spectrum. - Three surveys for measuring cosmic shear:
Stage three surveys:
a) Hyper-Supreme Cam Survey (HSC): Largest Telescope, 8 meter Subaru telescope, very deep and sharp images.
b) Dark Energy Survey (DES): Largest camera, covering a larger area, hence a large cosmological volume.
c) Kilo Degree Survey (KiDS) and the “sister survey” VIKING: A wider wavelength range from the near UV to the near IR. A dedicated weak-lensing telescope.
Each of these surveys will cover 100s of millions of galaxies -> large data sets. - Results of Stage III surveys:
Sigma8 vs. Omega matter.
All weak-lensing surveys agree with each other, but all of them give S8 lower than that from Planck. - Systematics:
Shape measurements: accurate ellipticities, correcting on PSF.
Redshifts distribution: potentially most serious systematic error.
Intrinsic alignments
Baryon feedback - The possible effect of redshift calibration on the S8 tension
Surveys have different approaches. - Constraints on S8:
3.2 sigma tension between weak-lensing and Planck. - KiDS-VIKING “gold” sample:
Identifying and rejecting galaxies with bad calibration, hence obtaining a more robust calibration. - Other probes:
“Not a single late Universe LSS measurements yields and S8 higher than Planck”. - Combining cosmic shear with other LSS probes
- Future prospects
- Summary
Nathalie Palanque-Delabrouille, Baryon Acoustic Oscillations with DESI
- Intro:
An intro to Baryon Acoustic Oscillations (BAO).
Baryons, dark matter (DM), and photons have an initial distribution in the early Universe. Because of Compton scattering, the baryons are carried over with photons, hence moving the baryon over-densities away from the DM over-densities. The characteristic length scale between the location of initial DM over-density and the distance that the baryons have been carried over by the photons at the recombination is defined as the sound horizon. A length scale of ~150 kpc at the recombination which, due to the expansion of the Universe, has evolved to a scale of ~150 Mpc today.
Galaxies are expected to form at the peaks of the density distribution, therefore, a same characteristic scale is expected to be observed in the typical (statistical) distances of galaxies in the sky. - Two-point statistics for measuring the BAO feature
Either correlation function in real-space (where peaks are expected to be seen) or power-spectrum in Fourier space (where wiggles are expected to be seen).
We now have over 8 sigma detection of the BAO feature (just from BOSS). - Comparing BAO surveys with Planck:
“Excellent agreement” - Relation to H0:
Anisotropic measurements of BAO:
a) transverse direction -> angular diameters distance
b) radial direction -> H(z) - Current results on BAO
- DESI:
Dark Energy Spectroscopic Instrument - DESI Targets:
Local Universe -> ~10 million very bright galaxies.
0.4 < z < 1.0 -> ~6 million luminous red galaxies (LRGs).
0.6 < z < 1.6 -> ~17 million emission line galaxies (ELGs).
larger redshifts -> ~2.4 million quasars.
in total -> around 35 million sources. - DESI hardware:
4 meter Mayall telescope at Kitt-peak in Arizona
FOV ~ 8 degree^2, sub-arcsec images, ten three-channel spectrographs, covering wavelengths ranging from UV to near IR, plus 5000 robotic fiber positioners. - BAO and H0:
H0 ~ 68 +/- 1.0 km/s.Mpc - Conclusions
BAO -> inverse distance ladder approach for model-dependent measurement of H0.
Julien Lesgourgues, The sound horizon from Big Bang Nucleosynthesis
- Intro:
H0 from early Universe observables other than H0. - Sound horizon angular scale (theta):
Theoretical relations with cosmological parameters. - H0 from angular & longitudinal BAO plus deuterium:
Combining the measurements of sound horizon angular scale at different redshifts.
H0 -> ~ 4 sigma tension with SH0ES. - How to get a larger H0 while complying with theta constraints?
- Possible connections with other tensions:
Sigma8 tension and small-scale problems of LCDM. - Conclusions
Vivian Poulin, Theoretical explanations/solutions
- Intro:
- How does CMB data measure H0?
- A varying dark energy density:
Good fit to Planck and SH0ES either with BAO or with SNIa, but the reconstructed expansions for the two are incompatible. Reason: BAO and SNIa measure the angular diameter and luminosity distances, respectively, and need to be calibrated for measuring H0. Different calibrators (Planck or local distance ladder) give different H0. “Unless you change the calibrator, you are creating new tension between SNIa and BAO”. - H0 tension or sound horizon tension?
- Early-time resolution to the H0 tension:
Exotic recombination.
With varying fundamental constants at the time of CMB the tension can be reduced (but not resolved).
Extra (dark) radiation and its interaction with dark matter!
Interacting neutrinos.
Early dark energy models and modified gravity theories (e.g. scalar-tensor theories). - Summary
Various models are proposed to resolve the tension -> no consensus yet.
M. Colless: Testing the BAO scale
“Maximum physics gain or minimum physics pain?“
P. Lemos: Remarks on possible theoretical resolutions.
L. Pogosian: It is hard to find a model that gets CMB+BAO to agree with direct H0 measurements.
A model X candidate.
N. G. Sanchez: Contributed comments/answers.
End of day four.
The images showing the first slide of each talk are screenshots obtained from the public YouTube video published by ESO. The featured image credit: M. Blanton and the Sloan Digital Sky Survey (SDSS).
