How do transients inform us about the Universe?

[jeffcooke]

A significant fraction of what we know about nature and the Universe has been discovered or informed by transients. This question addresses the large amount of work done, and that will be done in the next decade, that includes the following areas.

• Measuring the acceleration (and deceleration) of the Universe using Type Ia, Type IIP, kilonovae, SLSNe, and other transients to more accurately anchor the distance ladder footing, constrain dark energy, and that show promise to measure this parameter from the local Universe to the highest redshifts.
• Measuring the ionised baryon content in the Universe. Fast radio bursts (FRBs) are the only direct means to measure this quantity via their dispersion measure. This is a burgeoning field and requires large efforts for FRB detection, host galaxy localisation, and mapping the galaxies in the line of sight.
• Probing the ISM, CGM, and IGM of the Universe using transients as bright background beacons (e.g., gamma-ray bursts, SLSNe, etc.). These provide one of our only means to probe the very early Universe and to study their host proto-galaxies and line-of-sight environment. As they are temporary beacons, they provide a means at all redshifts to measure star formation and metallicity via absorption diagnostics during outburst and via emission diagnostics once they fade. They provide only means to directly calibrate these long-used galaxy measurement methods.
• Dark matter is accurately probed by lensed transient events using their time delays and require transient surveys.
• Measuring the chemical enrichment of the Universe over time requires the measured Type Ia and core-collapse supernovae rates from the local neighborhood to high redshift and even pair-instability supernova detections at early times, as they have a distinct chemical yield.
• The supernova rate from low to high redshift is an independent and consistent means to measure the cosmic star formation rate to help link the current multiple methods that are dependent on redshift.
• Transient rates and energies over cosmic time are essential to understand structure formation. For example, supernova feedback is one of the most important ingredients to understand galaxy formation and requires measured energies and rates of transients.
• Etc.

[ethrane]

@jeffcooke, I think this issue has a lot of overlap with https://github.com/ethrane/transients/issues/6; they are both general transient questions. I suggest that we move these bullets under that issue. Alternatively, you could pose the question differently in order to make this distinct from https://github.com/ethrane/transients/issues/6. (@kauchettl and I are going to merge similar issues in order to keep the discussions on each topic in a single place.) Thanks!

[jeffcooke]

Hi Eric,

Yes, one of the main aims of that question was to (1) hit on the large gap of things that is currently missing in the questions on the list (and some of the main reasons we do transient research) and, if we are only going to have 3-4 questions posed to the Town Hall, then (2) was put forth this as a more inclusive question to potentially merge others like "How does the sky change on human lifetimes? and How are the elements created? The first one is more of a curiosity and justification to discover things, new things, etc. the second is studying one of the key things we learn from transients.

The question I recently posed encompasses both of these, but includes many things they are missing or that lie outside of their scope, such as Type Ia cosmology (from the transients point of view, i.e., we need observations, theory on their progenitors, etc.), absorption-line neutral and ionised gas work (background beacons, FRB, SLSN, etc) and FRB ionised baryon content research, (again, from the transient research point of view, methods of detection, analysis, theory, host galaxies etc.), probing dark matter, transient impact on structure formation (again, from the transient perspective, discovery and follow up observations, energies from spectra, transient theory, etc.).

So, the most recent question poses a lot that not's in the others, but could include the others if the aim is for a single very broad question.

• Jeff

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From: ethrane @.> Sent: Monday, 11 March 2024 11:12 AM To: ethrane/transients @.> Cc: jeffcooke @.>; Mention @.> Subject: Re: [ethrane/transients] How do transients inform us about the Universe? (Issue #11)

@jeffcookehttps://github.com/jeffcooke, I think this issue has a lot of overlap with #6https://github.com/ethrane/transients/issues/6; they are both general transient questions. I suggest that we move these bullets under that issue. Alternatively, you could pose the question differently in order to make this distinct from #6https://github.com/ethrane/transients/issues/6. @.***https://github.com/kauchettl and I are going to merge similar issues in order to keep the discussions on each topic in a single place.) Thanks!

— Reply to this email directly, view it on GitHubhttps://github.com/ethrane/transients/issues/11#issuecomment-1987425473, or unsubscribehttps://github.com/notifications/unsubscribe-auth/AC5TFNGGB3HKTREPS3RRHDLYXTZFRAVCNFSM6AAAAABENV4QLKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTSOBXGQZDKNBXGM. You are receiving this because you were mentioned.Message ID: @.***>

[kauchettl]

Hi @jeffcooke!

As always, thank you so much for your insight and helpful comments/suggestions.

The plan is to share all of the proposed key questions in the town halls, so the community will see everything we discussed. And we will make it clear that they can propose their own questions too.

The first one is more of a curiosity and justification to discover things, new things, etc. the second is studying one of the key things we learn from transients.

I think this is a useful discussion––how to frame a broad question about transients––but I believe we should move this discussion to https://github.com/ethrane/transients/issues/6. When you have some time, do you mind posting this as a comment there so we can close this issue ?

Thank you so much!

[jeffcooke]

Hi Katie,

No worries. Are you and Eric asking me to move the newly added Key Question and place it under the question about the night sky varying? It's a different question, so t's unclear how the Town Hall people will see it. It would instead be a comment on Question #6. It's possible everyone attending the Town Hall will read all questions, comments, and discussions prior to meeting, but in most cases, unlikely.

Given that it's fundamentally a different question, it would be odd to put it as a comment to #6. That would be the similar to putting #2 and #3 together. Have a think about what each are actually asking. I'm trying to ensure that the questions are representing the community needs properly (not mine). Maybe the others can weigh in? I may be off the mark.

For example, if you want to measure the expansion rate of the Universe, would you include it under "How does the sky vary on human timescales? Or the ionised baryons over cosmic time? Or the star formation rate over cosmic time? Or you want to measure the dark matter mass of galaxies and galaxy clusters? Or measure the properties of GRB host galaxies. All these are legitimate work by Australians doing transient astronomy and will be very important in the next decade, with LSST, JWST, Roman and other facilities.

If they are asking the same thing (not sure how cosmic time is a human timescale), then which says it more accurately?

• Jeff

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From: kauchettl @.> Sent: Monday, 11 March 2024 12:41 PM To: ethrane/transients @.> Cc: jeffcooke @.>; Mention @.> Subject: Re: [ethrane/transients] How do transients inform us about the Universe? (Issue #11)

Hi @jeffcookehttps://github.com/jeffcooke!

As always, thank you so much for your insight and helpful comments/suggestions.

The plan is to share all of the proposed key questions in the town halls, so the community will see everything we discussed. And we will make it clear that they can propose their own questions too.

The first one is more of a curiosity and justification to discover things, new things, etc. the second is studying one of the key things we learn from transients.

I think this is a useful discussion––how to frame a broad question about transients––but I believe we should move this discussion to #6https://github.com/ethrane/transients/issues/6. When you have some time, do you mind posting this as a comment there so we can close this issue ?

Thank you so much!

— Reply to this email directly, view it on GitHubhttps://github.com/ethrane/transients/issues/11#issuecomment-1987480605, or unsubscribehttps://github.com/notifications/unsubscribe-auth/AC5TFNBDMC6Z7NJ73JWAXEDYXUDUNAVCNFSM6AAAAABENV4QLKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTSOBXGQ4DANRQGU. You are receiving this because you were mentioned.Message ID: @.***>

[kauchettl]

Hi Jeff,

Thanks for your further clarification. Re-reading over the discussion in this thread, my (maybe incorrect!) impression is that “How do transients inform us about the Universe” is similar to “How does the night sky (the universe) vary on human timescales” but just re-phrased differently. My impression is that they seem to be achieving the same goal of supporting transient astronomy in the community (vs. focusing on individual’s research interests).

However, as we want to involve the full WG in this decision, may I suggest you make a comment on https://github.com/ethrane/transients/issues/6 asking whether it would be better to change the phrasing of “How does the night sky (the universe) vary on human timescales” to “How do transients inform us about the Universe” and then link your discussion points? Since many people have already engaged/there has been lots of interaction with issue 6, I expect you will get more interaction/engagement given the context that you are suggesting.

If people agree they are the similar, then we can then change the name of the git issue #6 to “How does the night sky (the universe) vary on human timescales? / How do transients inform us about the Universe?” to make sure your suggestions and comments are not being lost, but if the wide WG think it should be a separate/new question then we can keep the git issue # 11 open and direct people to further add discussion there. How does this sound?

Thanks again!

~Katie and Eric

[adamdeller]

Somewhere (and I think it falls most naturally here) I would like to see the role of "standardisable" transients mentioned in the context of tools to probe fundamental physics and large scale structure. GW events, FRBs, Type Ia SNe all offer a standardisable "something" (and can be combined with lensing - or with each other!! - to give even more info). Jeff touched on this several times already. I feel like this is the part of this thread that is distinct from "How does the night sky (the Universe) vary on human timescales". The title of that other one sounds more like understanding the stuff that varies - what I'm then calling out as separate is using the (now understood) transients as tools.

[ethrane]

@adamdeller, I think a "transients in cosmology" would be good to suggest as a dedicated issue... maybe something like "What can astronomical transients teach us about the growth and history of the Universe?"

Focusing on cosmology would help make this question narrow enough to be useful when the decadal plan editors prioritise. At the same time, it's broad enough to include a lot of things that people in our community care about + create synergies with Group 1.1 Galaxies and Cosmology.

If you agree, can you please create a fresh issue to propose this?