Thermal history of the Universe at intermediate redshift: progress with 21cm absorption measurements

26 Jun 2018, 09:00 → 27 Jun 2018, 18:00 Europe/Zurich

4/3-006 - TH Conference Room (CERN)

Organisers : Kfir Blum, Guido D'Amico, Andi Hektor, Paolo Panci, Sergey Sibiryakov, Zvonimir Vlah

Tuesday 26 June

09:30 Coffee

1 An Absorption Feature in the Sky-Averaged Radio Spectrum

The EDGES experiment has detected with high S/N an absorption feature in the radio spectrum centered at 78 MHz. This feature is broadly consistent with expectations for the global 21-cm absorption signal from cosmic dawn. However, important aspects of the signal differ from physical predictions. In particular, 1) its absorption amplitude is larger than expected, 2) its profile has surprisingly sharp features, and 3) the center frequency is in tension with predictions from high-z UV galaxy luminosity functions. Understanding the key experimental aspects is crucial when interpreting this challenging measurement. In my talk I will describe the EDGES detection of this absorption feature using our two Low-Band instruments. I will discuss the lab and field calibrations, the verifications tests, the data analysis, and the parameter estimation. I will also summarize recent efforts to constrain traditional 21-cm models using EDGES High-Band data.

Speaker: Dr Raul Monsalve

Monsalve1.pdf

Monsalve2.pdf

2 Dark matter revealed by the first stars?

The EDGES global 21-cm experiment has detected stronger absorption than expected at cosmic dawn. This absorption can be explained by invoking excess cooling of the cosmic gas induced by an interaction with dark matter. This would have far reaching consequences, including an upper limit on the mass of dark matter particles that conflicts with the expectations for WIMPs. Specific particle physics models are highly constrained, but further observations will decide. In particular, we predict that 21-cm fluctuations at cosmic dawn are likely to be much larger than previously expected, and may exhibit a specific signature of the effect of dark matter.

Speaker: Prof. Rennan Barkana

Barkana.pdf

12:00 Lunch

15:00 Coffee

3 Bounds on Dark Matter annihilations from 21 cm data

The observation of an absorption feature in the 21 cm spectrum at redshift z ≈ 17 implies bounds on Dark Matter annihilations for a broad range of masses, given that significant heating of the intergalactic medium would have erased such feature. The resulting bounds on the DM annihilation cross sections are comparable to the strongest ones from all other observables.

Speaker: Dr Guido D'Amico

DAmico.pdf

4 Speculations on the EDGES 21cm anomaly

Possible attempts to resolve the EDGES anomaly rely on cooling cosmic hydrogen gas via interactions between dark and visible sector or modifying the soft photon background beyond CMB. The first solution is in tension with cosmological dark matter probes once simple dark sector models are considered. The second solution can be realised as soft photon emission from dark sector. However, the proposed mechanisms to produce soft photons need rather complex physics, e.g. axion miniclusters. The talk is based on our recent paper 1803.03245.

Speaker: Dr Andi Hektor

Hektor.pdf

5 Constraints on energy injection from 21-cm measurements: primordial black holes, axion-like particles and excited dark matter

I will review our recent papers 1803.09697 and 1805.09319

Speaker: Dr Gert Hutsi

Hutsi.pdf

Cocktail

Wednesday 27 June

09:30 Coffee

6 21 cm Cosmology: The status of the current redshifted 21 cm experiments

In the last decade a large effort has been dedicated to detect one of the last phase transitions in the Universe called the Epoch of Reionization and its preceding epoch know as the Cosmic Dawn, specially with redshifted 21 cm probes. I will review the current status of the various constraints that we currently have on reionization. I will also show the current results from the few redshifted 21 cm operating telescopes, a special
attention will be paid to the recent results obtained with EDGES and the LOFAR telescope. A discussion about the future of this effort will conclude my talk.

Speaker: Dr Saleem Zaroubi

Zaroubi.pdf

7 Probing non-cold dark matter models with the global 21-cm signal

The global 21-cm absorption signal induced by the first stars provides unique insights into structure formation at very high redshifts. I will show how the timing of such a signal can be used to obtain unprecedented constraints on the nature of dark matter.

Speaker: Dr Aurel Schneider

Schneider.pdf

12:00 Lunch

15:00 Coffee

8 Stronger 21cm absorption from charge sequestration

The unexpectedly strong 21cm absorption signal detected by the EDGES experiment suggests that the baryonic gas at the end of the dark ages was colder than predicted in the standard scenario. We discuss a mechanism to lower the baryon temperature after recombination. We introduce a stable, negatively-charged particle with a significant cosmological abundance, such that the universe remains charge-neutral but the electron and proton numbers are no longer equal. The deficit of electrons after recombination results in an earlier decoupling of the baryon and CMB temperatures, and thus in a colder gas at the cosmic dawn.

Speaker: Dr Adam Falkowski

Falkowski.pdf

9 The 21-cm signal and the high redshift IGM

The recent results of the EDGES experiment motivate us to take a closer look at the standard calculation of the 21cm signal. I will give a short introduction to the physics of the 21cm line, and how its brightness temperature depends on the physical conditions in the high redshift IGM, namely the local gas temperature and the flux of Lyman-alpha photons. I will then describe recent work that shows how the CMB heats the gas in the presence of the same Lyman-alpha photons that are needed to produce the signal. Time permitting, I will also touch on the implications of the shape of the EDGES signal for the fluctuations in the 21cm background from the same cosmological period.

Speaker: Dr Tejaswi Venumadhav

Venumadhav.pdf

10 21 cm physics as a probe of (a case for?) dark radiation.

Numerous quanta of soft dark radiation ( e.g. more abundant than CMB photons) could be created in the early Universe by means of various non-thermal processes. The dark radiation
can be resonantly converted to the "normal" photons, creating over-population of Rayleigh-Jeans photons, increasing the contrast of the 21 cm absorption feature. We construct specific classes of models to that effect, where the dark radiation in the form of the dark photons is sourced by the decay of sub-eV dark matter particles. We show that such models are perfectly consistent with all astrophysical, cosmological and terrestrial data, and 21 cm physics provides the leading constraint on it. In a more speculative vein, the strength of the EDGES signal can also be explained suing such a setup.

Speaker: Dr Maxim Pospelov

Pospelov.pdf