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SUMMARY:Defocusing dark energy: Raychaudhuri diagnostics beyond $w<-1/3$ a
 nd the phantom divide
DTSTART;VALUE=DATE-TIME:20260916T122500Z
DTEND;VALUE=DATE-TIME:20260916T125000Z
DTSTAMP;VALUE=DATE-TIME:20261028T221004Z
UID:indico-contribution-31298@indico.ific.uv.es
DESCRIPTION:Speakers: Ozgur Akarsu (Istanbul Technical University (ITU))\n
 In general relativity\, cosmic acceleration is a statement about geometry-
 --the timelike defocusing of the comoving congruence---and the field equat
 ions source it with mass--energy combinations\, not with any equation-of-s
 tate ratio. Evolving dark energy is now back at the center of cosmology\, 
 and several scenarios under active discussion---sign-switching $\\Lambda_{
 \\rm s}$CDM and AdS-to-dS transitions\, which simultaneously ease the $H_0
 $ and $S_8$ tensions and restore $\\sum m_\\nu$ to physical values\, as we
 ll as modified-gravity backgrounds recast in GR-like form---feature an eff
 ective dark-energy density that is negative in the past and crosses zero a
 t some redshift $z_\\dagger$. Our standard language quietly assumes this n
 ever happens: $w_{\\rm de}0$ and the inequality reverses on the negative b
 ranch\; the phantom divide $w_{\\rm de}=-1$ stops separating anything once
  the density changes sign\; at $\\rho_{\\rm de}=0$ the ratio $w_{\\rm de}$
  does not exist\, though nothing physical is singular there\; and sign-pre
 serving parametrizations such as $w_0$--$w_a$ cannot represent the crossin
 g at all. In this talk\, based on arXiv:2607.18008\, we ask what defines d
 ark energy when $w$ cannot\, and answer with the two combinations the fiel
 d equations actually use: the active gravitational mass density $\\mathcal
 {M}=\\rho+3p$\, which drives Raychaudhuri defocusing\, and the inertial ma
 ss density $\\mathcal{I}=\\rho+p$\, whose zero---the null energy condition
  (NEC) boundary---replaces the phantom divide as the regular separator. Sh
 arp results follow: any smooth sign switch is phantom-like and repulsive a
 t the crossing\, with the total NEC intact\; the pole in $w_{\\rm de}$ is 
 purely kinematic\, with universal residue $n(1+z_\\dagger)/3$\, so $w_{\\r
 m de}=-1$ is never attained and the two sides of the divide connect only t
 hrough the pole\; repulsion begins strictly before the switch\, at $z_{\\r
 m rep}>z_\\dagger$\; an exact window in $\\rho_{\\rm de}'(z_\\dagger)$ per
 mits acceleration at the crossing itself\; and the deceleration parameter 
 admits exactly one or three sign-changing zeros. An analytic $\\Lambda_{\\
 rm s}$CDM profile\, exponential infrared $f(T)$ gravity\, and the minimal 
 phantom brane realize the pattern. The practical moral for reconstructions
 : across $\\rho_{\\rm de}=0$ the regular targets are $(\\rho_{\\rm de}\,p_
 {\\rm de}\,\\mathcal{I}_{\\rm de}\,\\mathcal{M}_{\\rm de})$---kinematic to
 tals need no dark-sector split---and parametrizations should be allowed to
  change sign.\n\nhttps://indico.ific.uv.es/event/8160/contributions/31298/
LOCATION:
URL:https://indico.ific.uv.es/event/8160/contributions/31298/
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