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ohwilleke
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- TL;DR Summary
- A group of researchers has set forth an agenda for cosmology for the next decade worth considering.
In a series of four pre-prints a group of cosmology researchers has laid out an agenda for the next decade or so of research that is worth considering in connection with a Letter of Interest for the Snowmass 2021 conference (which may or may not end up being conducted in person in 2021) (the authors are the same for all four parts of the Letter of Interest).
They start from the paradigm assumption that the standard Λ Cold Dark Matter cosmological model provides an amazing description of a wide range of astrophysical and astronomical data, but nonetheless focus on emerging tensions between this model and observations that may require it to be modified or reconsidered.
Have they accurately summed up the issues cosmologists should be exploring?
Have they overstated the importance of any of these issues?
Have they overlooked any key issues?
Are their assumptions deeply flawed in any way?
Why?
One of the most encouraging aspects of the report is that it highlights how many independent serious effects are underway to investigate these questions. Astronomy and cosmology as a discipline has multiple focal points of cutting edge research compared to high energy physics that is dominated by the Large Hadron Collider and a few second tier experiments. These experiments keep the discipline grounded by providing a veritable firehose of new data to test existing theories in ways that weren't previously possible and to inform the development of new theories.
Cosmology Intertwined I: Perspectives for the Next Decade arXiv:2008.11283 [pdf]
Eleonora Di Valentino, Luis A. Anchordoqui, Yacine Ali-Haimoud, Luca Amendola, Nikki Arendse, Marika Asgari, Mario Ballardini, Elia Battistelli, Micol Benetti, Simon Birrer, Marco Bruni, Erminia Calabrese, David Camarena, Salvatore Capozziello, Angela Chen, Jens Chluba, Anton Chudaykin, Eoin Ó Colgáin, Francis-Yan Cyr-Racine, Paolo de Bernardis, Jacques Delabrouille, Jo Dunkley, Agnès Ferté, Fabio Finelli, Wendy Freedman, Noemi Frusciante, Elena Giusarma, Adrià Gómez-Valent, Will Handley, Luke Hart, Alan Heavens, Hendrik Hildebrandt, Daniel Holz, Dragan Huterer, Mikhail M. Ivanov, Shahab Joudaki, Marc Kamionkowski, Tanvi Karwal, Lloyd Knox, Luca Lamagna, Julien Lesgourgues, Matteo Lucca, Valerio Marra, Silvia Masi, Sabino Matarrese, Alessandro Melchiorri, Olga Mena, et al. (31 additional authors not shown)
The standard Λ Cold Dark Matter cosmological model provides an amazing description of a wide range of astrophysical and astronomical data. However, there are a few big open questions, that make the standard model look like a first-order approximation to a more realistic scenario that still needs to be fully understood. In this Letter of Interest we will list a few important goals that need to be addressed in the next decade, also taking into account the current discordances present between the different cosmological probes, as the Hubble constant H0 value, the σ8−S8 tension, and the anomalies present in the Planck results. Finally, we will give an overview of upgraded experiments and next-generation space-missions and facilities on Earth, that will be of crucial importance to address all these questions.
The body text of the introductory preprint containing the overview lays out more details (citations omitted) and provides a nice survey of coming events from observations already underway in the field:
The big questions and goals for the next decade – The standard Λ Cold Dark Matter (ΛCDM) cosmological model provides an amazing description of a wide range of astrophysical and astronomical data. Over the last few years, the parameters governing ΛCDM have been constrained with unprecedented accuracy by precise measurements of the cosmic microwave background (CM. However, despite its incredible success, ΛCDM still cannot explain key concepts in our understanding of the universe, at the moment based on unknown quantities like Dark Energy (DE), Dark Matter (DM) and Inflation. Therefore, in the next decade the first challenges would be to answer the following questions:
• What is the nature of dark energy and dark matter?
• Did the universe have an inflationary period? How did it happen? What is the level of non-gaussianities?
• Does gravity behave like General Relativity even at horizon size scales? Is there Modified Gravity?
• Do we need quantum gravity, or an unified theory for quantum field theory and General Relativity?
• Is the universe flat or closed?
• What is the age of the universe?
• Do we actually need physics beyond the Standard Model (SM) of particle physics?
• For each elementary particle, there is an antiparticle that has exactly the very same properties but opposite charge. Then, why we do not see antimatter in the universe?
• Will the swampland conjectures within string theory help with fine-tuning problems in cosmology? Alternatively, will cosmology help us observationally test conjectures from string theory?
The ΛCDM model can therefore be seen as an approximation to a more realistic scenario that still needs to be fully understood. However, since the ΛCDM model provides an extremely good fit of the data, deviations from the model are not expected to be too drastic from the phenomenological point of view, even if they can be conceptually really different. In particular, discrepancies with different statistical significance developing between observations at early and late cosmological time may involve the addition of new physics ingredients in the ΛCDM minimal model. For this reason, it is timely to investigate the disagreement at more than 4σ about the Hubble constant H(0), followed by the tension at ∼ 3σ on f(σ(8)) − S(8), and the anomalies in the Planck experiment results about the excess of lensing, the curvature of the Universe or its age. In the next decade we aim to address these discrepancies solving the following key questions:
• What is the origin of the sharpened tension in the observed and inferred values of H0, f σ8, and S8?
• Is it possible that some portion (with an outside chance of all) of the tension may still be systematic errors in the current measurements?
• Is the tension a statistical fluke or is it pointing to new physics?
• Is it possible to explain the tension without changing the standard ΛCDM cosmology?
• Is there an underlying new physics that can accommodate this tension?
In order to address all the open questions, and to change the ΛCDM from an effective model to a physical model, testing the different predictions, the goals for the next decade will be to:
• improve our understanding of systematic uncertainties;
• maximize the amount of information that can be extracted from the data by considering new analysis frameworks and exploring alternative connections between the different phenomena;
• improve our understanding of the physics on non-linear scales;
• de-standardize some of the ΛCDM assumptions, or carefully label them in the survey analysis pipelines, to pave the road to the beyond-ΛCDM models tests carried out by different groups.
This agenda is largely achievable in the next decade, thanks to a coordinated effort from the side of theory, data analysis, and observation. In separate LoI’s we provide a thorough discussion of these challenging questions, showing also the impossibility we have at the moment of solving all the tensions at the same time.
Stepping up to the new challenges – The next decade will provide a compelling and complementary view of the cosmos through a combination of enhanced statistics, refined analyses afforded by upgraded experiments and next-generation space-missions and facilities on Earth:
• Local distance ladder observations will achieve a precision in the H0 measurement of 1%.
• Gravitational time delays will reach a ∼ 1.5% precision on H0 without relying on assumption on the radial mass density profiles with resolved stellar kinematics measurement from JWST or the next generation large ground based extremely large telescopes (ELTs).
• CMB-S4 will constrain departures from the thermal history of the universe predicted by the SM. The departures are usually conveniently quantified by the contribution of light relics to the effective number of relativistic species in the early Universe, N(eff). CMB-S4 will constrain ∆Neff ≤ 0.06 at the 95% confidence level allowing detection of, or constraints on, a wide range of light relic particles even if they are too weakly interacting to be detected by lab-based experiments .
• The Euclid space-based survey mission will use cosmological probes (gravitational lensing, baryon acoustic oscillations (BAO) and galaxy clustering) to investigate the nature of DE, DM, and gravity.
• The Rubin Observatory Legacy Survey of Space and Time (LSST14) is planned to undertake a 10- year survey beginning in 2022. LSST will chart 20 billion galaxies, providing multiple simultaneous probes of DE, DM, and ΛCDM.
• The Roman Space Telescope (formerly known as WFIRST18) will be hundreds of times more efficient than the Hubble Space Telescope, investigating DE, cosmic acceleration, exoplanets, cosmic voids.
• The combination of LSST, Euclid, and WFIRST will improve another factor of ten the cosmological parameter bounds, allowing us to distinguish between models candidates to alleviate the tensions.
• The Square Kilometre Array (SKA) will be a multi-purpose radio-interferometer, with up to 10 times more sensitivity, and 100 times faster survey capabilities than current radio-interferometers, providing leading edge science involving multiple science disciplines. SKA will be able to probe DM properties (interactions, velocities and nature) through the detection of the redshifted 21 cm line in neutral hydrogen (HI), during the so-called Dark Ages, before the period of reionization. SKA will also be able to test the DE properties and the difference between some MG and DE scenarios by detecting the 21 cm HI emission line from around a billion galaxies over 3/4 of the sky, out to a redshift of z ∼ 2.
• CMB spectral distortions will be a possible avenue to test a variety of different cosmological models in the next decade, with applications ranging from non-standard inflationary scenarios and beyond the SM physics to the H0 tension;
• O(10^5 ) voids will be detected in upcoming surveys, that can place constraints on the expansion history of the universe following a purely geometric approach, and distinguish different gravity models .
• Gravitational wave (GW) coalescence events would provide a precise measurement of H(0). The LIGO-Virgo network operating at design sensitivity is expected to constrain H0 to a precision of ∼ 2% within 5 years and 1% within a decade. Moreover, even in absence of electromagnetic counterpart, it is possible to measure H0 cross-correlating with a clustering tracer, as a galaxy survey. Therefore, black hole binaries should provide a competitive H0 estimate fast.
• CERN’s LHC experiments ATLAS and CMS will provide complementary information by searching for the elusive DM particle and hyperweak gauge interactions of light relics. In addition, the ForwArd Search ExpeRiment (FASER) will search for light hyperweakly-interacting particles produced in the LHC’s high-energy collisions in the far-forward region.
Concluding, the current present tensions and discrepancies among different measurements, in particular the H0 tension as the most significant one, offer crucial insights in our understanding of the universe. For example, the standard distance ladder result has many steps in common with the accelerating universe discovery (which gave cosmology the evidence for DE). So, whatever the definite finding may be, whether about stars and their evolution, or DE, this is going to have far reaching consequences.
Cosmology Intertwined II: The Hubble Constant Tension arXiv:2008.11284 [pdf]
The current cosmological probes have provided a fantastic confirmation of the standard Λ Cold Dark Matter cosmological model, that has been constrained with unprecedented accuracy. However, with the increase of the experimental sensitivity a few statistically significant tensions between different independent cosmological datasets emerged. While these tensions can be in portion the result of systematic errors, the persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the need for new physics. In this Letter of Interest we will focus on the 4.4σ tension between the Planck estimate of the Hubble constant H0 and the SH0ES collaboration measurements. After showing the H0 evaluations made from different teams using different methods and geometric calibrations, we will list a few interesting new physics models that could solve this tension and discuss how the next decade experiments will be crucial.
Cosmology Intertwined III: fσ8 and S8 arXiv:2008.11285 [pdf]
The standard Λ Cold Dark Matter cosmological model provides a wonderful fit to current cosmological data, but a few tensions and anomalies became statistically significant with the latest data analyses. While these anomalies could be due to the presence of systematic errors in the experiments, they could also indicate the need for new physics beyond the standard model. In this Letter of Interest we focus on the tension of the Planck data with weak lensing measurements and redshift surveys, about the value of the matter energy density Ωm, and the amplitude or rate of the growth of structure (σ8,fσ8). We list a few interesting models for solving this tension, and we discuss the importance of trying to fit with a single model a full array of data and not just one parameter at a time.
Cosmology Intertwined IV: The Age of the Universe and its Curvature arXiv:2008.11286 [pdf]
A precise measurement of the curvature of the Universe is of primeval importance for cosmology since it could not only confirm the paradigm of primordial inflation but also help in discriminating between different early Universe scenarios. The recent observations, while broadly consistent with a spatially flat standard Λ Cold Dark Matter (ΛCDM) model, are showing tensions that still allow (and, in some cases, even suggest) a few percent deviations from a flat universe. In particular, the Planck Cosmic Microwave Background power spectra, assuming the nominal likelihood, prefer a closed universe at more than 99% confidence level. While new physics could be in action, this anomaly may be the result of an unresolved systematic error or just a statistical fluctuation. However, since a positive curvature allows a larger age of the Universe, an accurate determination of the age of the oldest objects provides a smoking gun in confirming or falsifying the current flat ΛCDM model.
They start from the paradigm assumption that the standard Λ Cold Dark Matter cosmological model provides an amazing description of a wide range of astrophysical and astronomical data, but nonetheless focus on emerging tensions between this model and observations that may require it to be modified or reconsidered.
Have they accurately summed up the issues cosmologists should be exploring?
Have they overstated the importance of any of these issues?
Have they overlooked any key issues?
Are their assumptions deeply flawed in any way?
Why?
One of the most encouraging aspects of the report is that it highlights how many independent serious effects are underway to investigate these questions. Astronomy and cosmology as a discipline has multiple focal points of cutting edge research compared to high energy physics that is dominated by the Large Hadron Collider and a few second tier experiments. These experiments keep the discipline grounded by providing a veritable firehose of new data to test existing theories in ways that weren't previously possible and to inform the development of new theories.
Cosmology Intertwined I: Perspectives for the Next Decade arXiv:2008.11283 [pdf]
Eleonora Di Valentino, Luis A. Anchordoqui, Yacine Ali-Haimoud, Luca Amendola, Nikki Arendse, Marika Asgari, Mario Ballardini, Elia Battistelli, Micol Benetti, Simon Birrer, Marco Bruni, Erminia Calabrese, David Camarena, Salvatore Capozziello, Angela Chen, Jens Chluba, Anton Chudaykin, Eoin Ó Colgáin, Francis-Yan Cyr-Racine, Paolo de Bernardis, Jacques Delabrouille, Jo Dunkley, Agnès Ferté, Fabio Finelli, Wendy Freedman, Noemi Frusciante, Elena Giusarma, Adrià Gómez-Valent, Will Handley, Luke Hart, Alan Heavens, Hendrik Hildebrandt, Daniel Holz, Dragan Huterer, Mikhail M. Ivanov, Shahab Joudaki, Marc Kamionkowski, Tanvi Karwal, Lloyd Knox, Luca Lamagna, Julien Lesgourgues, Matteo Lucca, Valerio Marra, Silvia Masi, Sabino Matarrese, Alessandro Melchiorri, Olga Mena, et al. (31 additional authors not shown)
The standard Λ Cold Dark Matter cosmological model provides an amazing description of a wide range of astrophysical and astronomical data. However, there are a few big open questions, that make the standard model look like a first-order approximation to a more realistic scenario that still needs to be fully understood. In this Letter of Interest we will list a few important goals that need to be addressed in the next decade, also taking into account the current discordances present between the different cosmological probes, as the Hubble constant H0 value, the σ8−S8 tension, and the anomalies present in the Planck results. Finally, we will give an overview of upgraded experiments and next-generation space-missions and facilities on Earth, that will be of crucial importance to address all these questions.
The body text of the introductory preprint containing the overview lays out more details (citations omitted) and provides a nice survey of coming events from observations already underway in the field:
The big questions and goals for the next decade – The standard Λ Cold Dark Matter (ΛCDM) cosmological model provides an amazing description of a wide range of astrophysical and astronomical data. Over the last few years, the parameters governing ΛCDM have been constrained with unprecedented accuracy by precise measurements of the cosmic microwave background (CM. However, despite its incredible success, ΛCDM still cannot explain key concepts in our understanding of the universe, at the moment based on unknown quantities like Dark Energy (DE), Dark Matter (DM) and Inflation. Therefore, in the next decade the first challenges would be to answer the following questions:
• What is the nature of dark energy and dark matter?
• Did the universe have an inflationary period? How did it happen? What is the level of non-gaussianities?
• Does gravity behave like General Relativity even at horizon size scales? Is there Modified Gravity?
• Do we need quantum gravity, or an unified theory for quantum field theory and General Relativity?
• Is the universe flat or closed?
• What is the age of the universe?
• Do we actually need physics beyond the Standard Model (SM) of particle physics?
• For each elementary particle, there is an antiparticle that has exactly the very same properties but opposite charge. Then, why we do not see antimatter in the universe?
• Will the swampland conjectures within string theory help with fine-tuning problems in cosmology? Alternatively, will cosmology help us observationally test conjectures from string theory?
The ΛCDM model can therefore be seen as an approximation to a more realistic scenario that still needs to be fully understood. However, since the ΛCDM model provides an extremely good fit of the data, deviations from the model are not expected to be too drastic from the phenomenological point of view, even if they can be conceptually really different. In particular, discrepancies with different statistical significance developing between observations at early and late cosmological time may involve the addition of new physics ingredients in the ΛCDM minimal model. For this reason, it is timely to investigate the disagreement at more than 4σ about the Hubble constant H(0), followed by the tension at ∼ 3σ on f(σ(8)) − S(8), and the anomalies in the Planck experiment results about the excess of lensing, the curvature of the Universe or its age. In the next decade we aim to address these discrepancies solving the following key questions:
• What is the origin of the sharpened tension in the observed and inferred values of H0, f σ8, and S8?
• Is it possible that some portion (with an outside chance of all) of the tension may still be systematic errors in the current measurements?
• Is the tension a statistical fluke or is it pointing to new physics?
• Is it possible to explain the tension without changing the standard ΛCDM cosmology?
• Is there an underlying new physics that can accommodate this tension?
In order to address all the open questions, and to change the ΛCDM from an effective model to a physical model, testing the different predictions, the goals for the next decade will be to:
• improve our understanding of systematic uncertainties;
• maximize the amount of information that can be extracted from the data by considering new analysis frameworks and exploring alternative connections between the different phenomena;
• improve our understanding of the physics on non-linear scales;
• de-standardize some of the ΛCDM assumptions, or carefully label them in the survey analysis pipelines, to pave the road to the beyond-ΛCDM models tests carried out by different groups.
This agenda is largely achievable in the next decade, thanks to a coordinated effort from the side of theory, data analysis, and observation. In separate LoI’s we provide a thorough discussion of these challenging questions, showing also the impossibility we have at the moment of solving all the tensions at the same time.
Stepping up to the new challenges – The next decade will provide a compelling and complementary view of the cosmos through a combination of enhanced statistics, refined analyses afforded by upgraded experiments and next-generation space-missions and facilities on Earth:
• Local distance ladder observations will achieve a precision in the H0 measurement of 1%.
• Gravitational time delays will reach a ∼ 1.5% precision on H0 without relying on assumption on the radial mass density profiles with resolved stellar kinematics measurement from JWST or the next generation large ground based extremely large telescopes (ELTs).
• CMB-S4 will constrain departures from the thermal history of the universe predicted by the SM. The departures are usually conveniently quantified by the contribution of light relics to the effective number of relativistic species in the early Universe, N(eff). CMB-S4 will constrain ∆Neff ≤ 0.06 at the 95% confidence level allowing detection of, or constraints on, a wide range of light relic particles even if they are too weakly interacting to be detected by lab-based experiments .
• The Euclid space-based survey mission will use cosmological probes (gravitational lensing, baryon acoustic oscillations (BAO) and galaxy clustering) to investigate the nature of DE, DM, and gravity.
• The Rubin Observatory Legacy Survey of Space and Time (LSST14) is planned to undertake a 10- year survey beginning in 2022. LSST will chart 20 billion galaxies, providing multiple simultaneous probes of DE, DM, and ΛCDM.
• The Roman Space Telescope (formerly known as WFIRST18) will be hundreds of times more efficient than the Hubble Space Telescope, investigating DE, cosmic acceleration, exoplanets, cosmic voids.
• The combination of LSST, Euclid, and WFIRST will improve another factor of ten the cosmological parameter bounds, allowing us to distinguish between models candidates to alleviate the tensions.
• The Square Kilometre Array (SKA) will be a multi-purpose radio-interferometer, with up to 10 times more sensitivity, and 100 times faster survey capabilities than current radio-interferometers, providing leading edge science involving multiple science disciplines. SKA will be able to probe DM properties (interactions, velocities and nature) through the detection of the redshifted 21 cm line in neutral hydrogen (HI), during the so-called Dark Ages, before the period of reionization. SKA will also be able to test the DE properties and the difference between some MG and DE scenarios by detecting the 21 cm HI emission line from around a billion galaxies over 3/4 of the sky, out to a redshift of z ∼ 2.
• CMB spectral distortions will be a possible avenue to test a variety of different cosmological models in the next decade, with applications ranging from non-standard inflationary scenarios and beyond the SM physics to the H0 tension;
• O(10^5 ) voids will be detected in upcoming surveys, that can place constraints on the expansion history of the universe following a purely geometric approach, and distinguish different gravity models .
• Gravitational wave (GW) coalescence events would provide a precise measurement of H(0). The LIGO-Virgo network operating at design sensitivity is expected to constrain H0 to a precision of ∼ 2% within 5 years and 1% within a decade. Moreover, even in absence of electromagnetic counterpart, it is possible to measure H0 cross-correlating with a clustering tracer, as a galaxy survey. Therefore, black hole binaries should provide a competitive H0 estimate fast.
• CERN’s LHC experiments ATLAS and CMS will provide complementary information by searching for the elusive DM particle and hyperweak gauge interactions of light relics. In addition, the ForwArd Search ExpeRiment (FASER) will search for light hyperweakly-interacting particles produced in the LHC’s high-energy collisions in the far-forward region.
Concluding, the current present tensions and discrepancies among different measurements, in particular the H0 tension as the most significant one, offer crucial insights in our understanding of the universe. For example, the standard distance ladder result has many steps in common with the accelerating universe discovery (which gave cosmology the evidence for DE). So, whatever the definite finding may be, whether about stars and their evolution, or DE, this is going to have far reaching consequences.
Cosmology Intertwined II: The Hubble Constant Tension arXiv:2008.11284 [pdf]
The current cosmological probes have provided a fantastic confirmation of the standard Λ Cold Dark Matter cosmological model, that has been constrained with unprecedented accuracy. However, with the increase of the experimental sensitivity a few statistically significant tensions between different independent cosmological datasets emerged. While these tensions can be in portion the result of systematic errors, the persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the need for new physics. In this Letter of Interest we will focus on the 4.4σ tension between the Planck estimate of the Hubble constant H0 and the SH0ES collaboration measurements. After showing the H0 evaluations made from different teams using different methods and geometric calibrations, we will list a few interesting new physics models that could solve this tension and discuss how the next decade experiments will be crucial.
Cosmology Intertwined III: fσ8 and S8 arXiv:2008.11285 [pdf]
The standard Λ Cold Dark Matter cosmological model provides a wonderful fit to current cosmological data, but a few tensions and anomalies became statistically significant with the latest data analyses. While these anomalies could be due to the presence of systematic errors in the experiments, they could also indicate the need for new physics beyond the standard model. In this Letter of Interest we focus on the tension of the Planck data with weak lensing measurements and redshift surveys, about the value of the matter energy density Ωm, and the amplitude or rate of the growth of structure (σ8,fσ8). We list a few interesting models for solving this tension, and we discuss the importance of trying to fit with a single model a full array of data and not just one parameter at a time.
Cosmology Intertwined IV: The Age of the Universe and its Curvature arXiv:2008.11286 [pdf]
A precise measurement of the curvature of the Universe is of primeval importance for cosmology since it could not only confirm the paradigm of primordial inflation but also help in discriminating between different early Universe scenarios. The recent observations, while broadly consistent with a spatially flat standard Λ Cold Dark Matter (ΛCDM) model, are showing tensions that still allow (and, in some cases, even suggest) a few percent deviations from a flat universe. In particular, the Planck Cosmic Microwave Background power spectra, assuming the nominal likelihood, prefer a closed universe at more than 99% confidence level. While new physics could be in action, this anomaly may be the result of an unresolved systematic error or just a statistical fluctuation. However, since a positive curvature allows a larger age of the Universe, an accurate determination of the age of the oldest objects provides a smoking gun in confirming or falsifying the current flat ΛCDM model.
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