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 three 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 here for a summary of day 1 and here for summary of day 2.
After Sherry Suyu announced the start of the third day, the talks went on as follows:
Tamara Davis, On redshift measurements
- Intro:
Are there any uncertainties in redshift, z, that we should worry about?
“Could z errors be biasing H0? Yes, but unlikely to be sufficient to explain the H0 tension”, but “there are common errors in z that we need to fix”. - How large a redshift error would matter?
At low-z it could make a difference.
For keeping H0 uncertainty low, we need to control systematics at the level of 0.0001. More details in David et al. (2019). - What does a 0.0001 redshift error look like?
- Possible sources of redshift bias:
a) Observational errors:
Systematic ones are important e.g. spectrograph wavelength calibration, line smoothing, air to vacuum conversion, and rest frame wavelength calibration.
b) Physical effects:
Gravitational redshift measured by observer in cluster or void (small error).
Peculiar velocities can cause large errors:
Sun around the MW ~0.001 in z, MW w.r.t. CMB ~0.002 in z.
Also peculiar velocities of the distance galaxies could cause problems.
Peculiar velocity correction is important for H0 from GW:
But this will cancel out as the number of sources increases (different sources have different random peculiar velocities).
c) Theoretical error!:
*Adding redshifts*
Checking SNeIa redshift samples - Bulk flow overcorrection:
Nearby galaxies share some of our motion w.r.t. the CMB. - Candles vs. Rulers?
- Conclusions
Kenneth Wong, H0 from strong gravitational lenses
- Intro:
Intro to gravitational lensing.
The lens produces multiple images of the same background source.
The lensing effect depends on the mass distribution of the lens, line of sight structure, and cosmology (hence H0). - Time-delay cosmography:
A time-delay between different lensed images of a variable source due to different path lengths. This is related to the time delay distance and “Fermat potential” from the lens model.
Therefore, by measuring the time delay and with a lens model, one can get a distance and then H0. - Cosmology with lensed quasars:
Obtaining light curves, detect features corresponding to same source events, but shifted in time. - Improvements on time-delay cosmography
- H0LiCOW:
H0 Lenses in COSMOGRAIL’s Wellspring.
Analyzed 7 lenses so far. - Time-delay Measurements:
Long-term monitoring of several lensed quasars with 1-2 m telescopes. - Lens modeling:
Using deep HST and adaptive optics imaging.
High resolution is needed.
The velocity dispersion of the lens galaxy is needed. - Mass along the line of sight:
Lenses tend to lie in an overdense line of sight. - Blind Analysis:
H0 is blinded throughout the analysis to avoid confirmation bias. - H0 from lensing:
73.3 +1.7 / -1.8 km/s.Mpc - Future of time-delay cosmography
- Summary
Dominic Pesce, The Megamaser Cosmology Project
- Intro:
A brief review on what magamaser systems are and how they are used for measuring H0.
Using masers that are in AGN accretion disks and are observed in radio.
NGC 4258: the first discovered disk magamaser. - Disk Masers – basic model:
There are groups of maser features: blueshifted, redshifted, and systemic.
Maser disk is edge-on: position, line of sight velocity, and line of sight acceleration are observed, then the black hole mass and the distance to the system are inferred. - Controlling the systematics
- The Megamaser Cosmology Project (MCP):
Advantage: The megamaser method gives a geometrical distance.
Disadvantage: Megamasers are rare systems.
Distance to five megamaser host galaxies so far. - H0 from MPC:
73.9 +/- 3 km/s.Mpc - Sources of uncertainties:
Systematic: peculiar velocities.
Statistical: quality of distance measurements and the small sample size (currently only 6 sources). - Summary
Bernard Schutz, Standard Sirens
- Intro:
H0 from standard sirens.
Bright sirens:
First GW BNS: GW170817, localized and identified, distance estimated, H0 measured 69 +22/ -8 km/s.Mpc. - GW H0 updates:
Early data on the GRB170817: H0=74 + 11.5 / -7.5 km/s.Mpc
Late-time GRB170817 superluminal motion: H0=70.3 +5.3 / -5.0 km/s.Mpc.
Using only dark sirens (binary blackhole mergers with no clear counterparts): H0= 68 +14 / -7 km/s.Mpc.
New results from GW190814: H0= 70 +17 / -8 km/s.Mpc. - Standard Sirens
- The sources of uncertainties:
Too few detectors.
Detector calibration.
Determining inclination: the detection is biased towards face-one systems.
Weak lensing: Relevant on cosmological scales and important for LISA. - Event Numbers
- Longer term prospect:
Together with ATHENA, LISA (which will launch around 2034) could be a very powerful probe of H(z).
S. Suyu, S. Birrer, R. Anderson, and O. Lahav
Peculiar velocities
How to increase the sample sizes (masers, sirens, lenses)?
How good is the localization of SMBH with LISA?
Going open source, publishing every code and data to allow reproducibility
On averaging H0 from different methods.
End of day three.
The images showing the first slide of each talk are screenshots obtained from the public YouTube video published by ESO. The featured image credit: NASA, ESA, S.H. Suyu (Max Planck Institute for Astrophysics, Technical University of Munich, and Academia Sinica Institute of Astronomy and Astrophysics), and K.C. Wong (University of Tokyo’s Kavli Institute for the Physics and Mathematics of the Universe).
