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SUMMARY:Tangled $τ$ and Flash Reionization
DTSTART;VALUE=DATE-TIME:20261028T180500Z
DTEND;VALUE=DATE-TIME:20261028T181500Z
DTSTAMP;VALUE=DATE-TIME:20261024T010901Z
UID:indico-contribution-3631@nyx.physics.mcgill.ca
DESCRIPTION:Speakers: Param Upadhyay (UManitoba)\nRecent cosmological tens
 ions have renewed interest in the role of reionization in cosmological par
 ameter inference. In particular\, “flash reionization” has been propos
 ed as a transient episode of ionization at high redshift that could increa
 se the Thomson optical depth\, $\\tau$\, while remaining compatible with C
 MB polarization measurements.\n\nIn this work\, we present the first Marko
 v Chain Monte Carlo analysis of flash reionization using Planck CMB temper
 ature\, polarization\, and lensing data. Using Planck PR4\, we find that t
 he CMB permits a wide range of flash redshifts\, with the allowed redshift
  strongly correlated with the peak ionization fraction. For benchmark scen
 arios at $z_{\\rm flash}=20$ and $25$\, we obtain $x_{\\rm flash}=0.24^{+0
 .13}_{-0.07}$ and $0.15^{+0.07}_{-0.05}$\, respectively.\n\nDespite this a
 dditional freedom in the ionization history\, the standard $\\Lambda$CDM p
 arameters remain largely unchanged. In particular\, we find $\\tau = 0.061
 \\pm0.007$ \ncompared with $\\tau=0.059\\pm0.006$ for standard tanh reioni
 zation. Interestingly\, repeating the analysis with Planck PR3 removes the
  mild preference for a flash and instead yields upper limits on its amplit
 ude. I will discuss what these results tell us about how much non-standard
  reionization history can be hidden within current CMB data and the implic
 ations for cosmological inference.\n\nhttps://nyx.physics.mcgill.ca/event/
 587/contributions/3631/
LOCATION:Online 
URL:https://nyx.physics.mcgill.ca/event/587/contributions/3631/
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