Cosmology wH0rksH0p

17 Nov 2024, 15:00 → 21 Nov 2024, 12:00 Europe/Zurich

Holckenhavn slot

Martin Snoager Sloth (Universe-Origins, University of Southern Denmark (DK)), Florian Niedermann (CP3-Origins / University of Southern Denmark)

Something is rotten in the state of cosmology. To be, or not to be, that is the question: Do we require an extension of L-CDM, and if so, which form should it take? We meet at a Danish castle to discuss these existential questions in cosmology.

 

Scope

The Hubble tension has risen to become one of cosmology’s most hotly debated topics. It calls for new physics in the early Universe at energy scales that can be probed with observations of the cosmic microwave background. However, despite the fact that the tension has been  debated for close to a decade, there is only a small handful of interesting theoretical resolutions that offer both a solid micropyhsical  justification and a viable phenomenology. The aim of the wH0rksH0p is simple: Discuss the status of these existing solutions and find new ways forward.   

(Download a picture with higher quality here.)

 

Venue and Travel

Holckenhavn Slot -  Holckenhavn 1, 5800 Nyborg, Denmark

Arrivial by train from Copenhagen Airport (CPH):

1. Train from CPH airport to Nyborg main station (ca. 90 - 105 minutes, DSB: CPH Lufthavn til Nyborg St.)
2. Taxi to Holckenhavn (ca 10 minutes, order by phone: +45 - 70 10 33 20)

 

Participants

• Torsten Bringmann (University of Oslo)
• Aleksandr Chatrchyan  (Nordita)
• Guido d'Amico (University of Parma) 
• Mathias Garny (Technical University of Munich)
• Alex Kehagias (University of Athens)
• Florian Niedermann (Nordita)
• Henrique Rubira (Technical University of Munich)
• Théo Simon (Laboratoire Univers & Particules de Montpellier) 
• Martin S. Sloth (University of Southern Denmark)
• Thomas Tram (Aarhus University) 
• Radosław Wojtak (University of Copenhagen) 

 

Local guests from SDU:

• Mads Frandsen (University of Southern Denmark)
• Manuel Meyer (University of Southern Denmark)
  
  

Gender consideration: Female speakers were invited but rejected our invitations due to busy schedules.

Schedule:

Registration:

Due to limited capacity, there is no open registration for this event. If you want to particpate please reach out to the organizers. 

 

Group.jpg

Monday 18 November

1 From dark SU(N) to H0 and back again

Speaker: Mathias Garny (Technische Universitaet Muenchen (DE))

talk.pdf

10:00 Coffee

2 Type Ia supernova standardisation: taboo systematics in the local Hubble constant measurement

My talk will focus on unresolved problems in the current models of type Ia supernova standardisation and how they bias the Hubble constant determination. I will show that the supernova standardisation model employed in the SH0ES measurement leaves unaccounted for residuals in the calibration data (supernovae observed in host galaxies of Cepheids). These residuals can be traced back to the model's assumption that intrinsic and extrinsic (extinction in supernova host galaxies) supernova properties are identical in the calibration galaxies and in the Hubble flow. This, however, cannot be reconciled with a naturally expected bias resulting from selecting highly star forming and dust-rich calibration galaxies for observations of Cepheids. I will show that the selection problem requires a more sophisticated modelling of type Ia supernovae in which we account for supernova subpopulations related to their host galaxy environments. I will discuss results from a newly developed hierarchical Bayesian modelling type Ia supernovae and its impact on the Hubble constant determination. I will demonstrate that the new approach lowers the best fit Hubble constant to 70 km/s/Mpc, reducing the Hubble tension by a factor of 2, primarily due to stronger extinction in the calibration galaxies than in the Hubble flow -- the property which turns out to be the main cause of apparent residuals in the original modelling from SH0ES.

Speaker: Radosław Wojtak

wojtak-nyborg-11-2024.pdf

Short discussion session / Systematics or new models

What is the status/systematics of the distance ladder independent measurements?

How should we interpret the new results from Wendy Freedman et al. (CCHP)?

HNEDE: Other dark radiation models (step model, decaying DM->DR, interacting DM-DR) Signatures in LSS that can discriminate? Status w.r.t. H0 tensions?

3 Discussion session I / Systematics or new models

What is the status/systematics of the distance ladder independent measurements?

How should we interpret the new results from Wendy Freedman et al. (CCHP)?

HNEDE: Other dark radiation models (step model, decaying DM->DR, interacting DM-DR) Signatures in LSS that can discriminate? Status w.r.t. H0 tensions?

12:00 Lunch

4 On the implications of the cosmic calibration tension beyond H0 and the synergy between early- and late-time new physics

The cosmic calibration tension is a > 5σ discrepancy between the cosmological distance ladder built from baryonic acoustic oscillations calibrated by the Planck/ΛCDM sound horizon and Type Ia supernovae calibrated with the SH0ES absolute magnitude. In this talk, I will emphasize the consequences of this tension beyond the value of the Hubble constant, and the implications for physics beyond ΛCDM. First, I will show that the SH0ES calibration implies (in addition to a higher value of H0) a larger physical matter density, a larger clustering amplitude S8, as well as a lower age of the universe. Second, I will talk about the role of early- and late-time new physics in resolving all these discrepancies.

Speaker: Théo Simon

wH0rksH0p_Theo_SIMON.pdf

15:00 Coffee

5 Decaying dark matter and emulating CMB codes

In the last few years, advances in artificial neural networks has allowed fast and accurate emulation of cosmic microwave background (CMB) observables and, to a lesser degree, large-scale structure observables. The potential speed-up of this approach is significant: the execution time of a CMB code such as CLASS is of the order 10 core-seconds, while the output of the neural network is of the order 0.1 core-seconds. However, building the neural network in the first place for a new model may erode the benefits entirely. In this talk I will discuss the framework CONNECT that we have developed exemplified using decaying dark matter models. I will also discuss profile likelihoods as a tool for discovering volume effects in Bayesian posteriors.

Speaker: Dr Thomas Tram

Discussion / Cosmic tensions

Scope: Discuss the status of the cosmic tensions

Questions:

Could systematics still “solve” the Hubble tension, or are we at a point where new physics is needed?

How should we interpret the new results from Wendy Freedman et al?
Given the many future probes, and the fact that Bayesian analyses suffer from prior effects, what is the best way to actually assess if there are tensions (barring systematics)?

What can we learn from recently revealed systematics in type Ia data used in DESI constraints on time varying DE equation of state ?

What can/should make an observational anomaly a reliable candidate for cosmological signal ?

What is the best strategy to assess the potential for learning about cosmology from future anomalies in systematics-driven measurements ?

What is the timeline for e.g. 1% determination of H0 from probes such as standard sirens and strongly lensed quasars?

What is better NPIPE or Plik for CMB?

Can lab-based particle physics (accelerator physics, neutrino detectors, dark matter detectors, etc.) be relevant for understanding the solutions to cosmic tensions, and if so, how?

Tuesday 19 November

6 Beyond decay: late-time conversion of dark matter to invisible radiation

In the cosmological concordance model, dark matter is assumed to be cold,
non-interacting and covariantly conserved. An interesting possibility to
consider is a violation of the third assumption in this simple picture,
namely the conversion of a small fraction of dark matter to an invisible
form of radiation.

So far, surprisingly little attention has been given to this option without
theoretical bias towards a specific scenario like decaying dark matter.
Here I will discuss how cosmic microwave and large-scale structure
observations can probe dark matter conversion in a model-independent way,
thus putting a conservative bound on how much dark matter could have
disappeared at any point during the cosmological evolution. For late
conversion times, but still before the onset of structure formation,
such a 'disappearance' of a few percent of dark matter would even
mitigate some of the well-known tensions between these datasets.

There is a variety of more concrete scenarios that can be mapped to this
general idea, such as decaying dark matter or merging primordial black holes.
In the second part of the talk, I will discuss yet another concrete particle
physics realization, featuring a second era of dark matter annihilation
after thermal freeze-out. As a bonus, this model naturally allows for
velocity-dependent dark matter self-interactions strong enough to affect
structure formation at dwarf galaxy scales in potentially observable ways.

Speaker: Torsten Bringmann (University of Oslo (NO))

Bringmann_wH0rksH0p.pdf

10:00 Coffee

Walk and talk in the castle park

What are the interesting questions in BH physics?
Is there any overlap with questions relating to the Hubble tension? (e.g. primordial BHs from first-order phase transition)

Status of standard sirens?

Probing new physics related to the Hubble tension with BH binaries? (e.g. DM physics, superradiance as a probe of new light degrees of freedom)

12:00 Lunch

7 TBA

Speaker: Mads Frandsen

15:00 Coffee

8 Probing the intergalactic magnetic field through gamma-ray ob- servations

Magnetic fields in galaxies and galaxy clusters are believed to be the result of the amplification of seed fields during structure formation. However, the origin of this intergalactic magnetic field (IGMF) remains unknown. Observations of high-energy gamma rays from distant sources offer an in-direct probe of the IGMF. Gamma-rays interact with the extragalactic background light to produce electron-positron pairs, which can subsequently initiate electromagnetic cascades whose gamma-ray signature depends on the IGMF. The absence of the cascade signal has been used to place lower
bounds on the IGMF. In this overview, I will introduce the method of how to search for this cascade emission using spectral, spatial, and timing information of the signal, and review recent results and highlights. I will also discuss sources of uncertainties regarding predictions of the cascade and provide an outlook for the potential of future gamma-ray observations, in particular with the upcoming Cherenkov Telescope Array

Speaker: Manuel Meyer

MMeyer_CosmicMF_Nyborg_2024.pdf

Discussion / New physics

Scope: Discuss the prospects of new physics under the assumption that the H0 tension is robust within LCDM )

Questions:

What are currently the most promising/natural/interesting solutions to the H0 tension?

How can one best observationally test if it is new recombination physics (modification of recombination history) or if it is new pre-recombination physics (e.g. energy injection)?

Are there other promising early-time approaches that do not rely on shortening the sound horizon?

What is the role of late-time new physics in resolving the Hubble tension?

Could the solution reside in the visible sector alone?

How can we use LSS/LyAlpha observations to learn about new physics related to the H0 tension?

What are the other important probes of new physics related to the H0 tension?

Which complementary signatures do models addressing the H0 tension predict? (e.g. stochastic grav. waves, DM-DR interaction, Lyman-alpha, galaxy clustering) Can we use them to discriminate and maybe identify or rule out certain scenarios?

How do the questions about DM relate to the H0 tension (interactions, small scales, production, etc..)?

How do the questions about DE relate to the H0 tension (e.g. quintessence or CC)?

What other types of new physics discussed in cosmology could relate to the Hubble tension (e.g. ALPs, phase transitions, modified gravity)?

Wednesday 20 November

9 The EFTofLSS: going forward

Speaker: Guido D'Amico (Universita degli Studi di Parma (IT))

10:00 Coffee

10 The Renormalization group of Large Scale Structure

Speaker: Henrique Rubira

Short discussion session / Large-scale structure

In which way can LSS tools be leveraged beyond the current state of the art to teach us more about the tensions in the future?

Why do models that resolve the Hubble tension often worsen the S8 tension?

Lyman-alpha measurements have a disputed history. Do we trust them now and what do they tell us?

Are the S8 tension and the “age problem” sufficiently significant to care about? If not, why? If yes, what could they be telling us?

12:00 Lunch

11 The Hubble Tension as the Signature of a New Phase of Dark Energy

In my talk, will argue that the Hubble tension might be the signature of new physics in the early Universe. First, I will describe the conditions new physics has to satisfy to stand a chance of addressing the Hubble tension without compromising the fit of the CMB. Secondly, I will argue that this new physics could be related to a new phase of dark energy that decays in a first-order phase transition before the CMB forms. In particular, I will consider the model of Cold New Early Dark Energy and highlight the central role of an ultralight scalar field that triggers the phase transition and contributes a small fraction of fuzzy dark matter.

Speaker: Florian Niedermann (Nordita)

NEDE.pdf

15:00 Coffee

12 New early dark energy and its equation of state

Cold New Early Dark Energy (NEDE) addresses the Hubble tension using a triggered vacuum phase transition in the dark sector. In this work we constrain the phenomenological fluid model properties of NEDE using recent datasets. We allow the equation of state parameter, characterizing the post-phase transition fluid, to evolve in time. We discuss simple physical scenarios where such a time-dependence can arise.

Speaker: Aleksandr Chatrchyan

Discussion / Generation alpha questions of cosmology

Scope: What are the most important big questions of cosmology? How do we hope to make progress? Is the H0 tension a good guide towards it?

Questions:
Will the cosmic tensions bring us closer to an understanding of dark energy, the cosmological constant problem, the origin of dark matter, or other fundamental problems in cosmology?

Does a modification of gravity in the IR play a role?

How reliable are the LCDM-inferred initial conditions of the Universe? In particular, will the Hubble tension change our understanding of the very early universe and inflation?

Is there a connection to the origin of neutrino masses?

Is there a connection to quantum gravity (axiverse, swampland)?

Is the H0 tension a window into the dark sector, and what can we learn about dark matter? Dark energy? Neutrinos? Other?

Which of these questions (if any) are we guaranteed to find an answer to?