Gravitational lensing and Dark Energy

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Discussion Overview

The discussion centers on the relationship between gravitational lensing and dark energy, particularly how gravitational lensing impacts the Cosmic Microwave Background (CMB) power spectra and its implications for understanding dark energy. Participants explore theoretical aspects, observational consequences, and the interplay between gravitational lensing and dark energy measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that gravitational lensing affects CMB power spectra for high values of l, questioning how this relates to the Integrated Sachs-Wolfe effect, which is said to affect low values of l.
  • Another participant explains that gravitational lensing mixes polarization signals, causing E-mode power to appear in B-modes, particularly at higher l values.
  • A participant inquires about the usefulness of gravitational lensing in determining features of dark energy.
  • Responses indicate that while dark energy's effect on the CMB is small, gravitational lensing provides measurements of large scale structure, which can inform about the universe's expansion and thus dark energy.
  • One participant challenges the assertion that lensing has little to do with dark energy, citing Planck's ability to constrain dark energy using lensing data.
  • Another participant argues that the main constraints on dark energy come from the Integrated Sachs-Wolfe effect rather than lensing directly.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between gravitational lensing and dark energy, with some asserting that lensing has minimal direct impact on dark energy measurements, while others argue that it can provide constraints through large scale structure observations. The discussion remains unresolved regarding the extent of lensing's influence on dark energy.

Contextual Notes

Participants highlight the complexity of the relationship between gravitational lensing, the CMB, and dark energy, noting that assumptions about the effects of lensing and the ISW effect may vary. The discussion reflects ongoing debates in the field regarding the interpretation of observational data.

astrolollo
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Hi everyone
I have been told that gravitational lensing affects CMB power spectra (TT, TE, EE) for high values of l (i.e. the least, low peaks on the right, say l>1000). But how? Isn't the effect of the varying gravitational potential along the line of sight the cause of Integrated Sachs-Wolfe effect which affects the spectrum, on the contrary, for low values of l (say, l< 30)? How can we know that the lensing is caused in an accelerating universe scenario? And how can gravitational lensing constrain DE, both in density and in equation of state?
 
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The primary impact of gravitational lensing is that it "mixes" the polarization signal, so that some of the E-mode power shows up in the B-modes (the primordial B-modes are much, much smaller than the E-modes, so there isn't much contamination in the other direction). Lensing shows up at higher ##\ell## values in the B-modes because the E-modes have more power at higher ##\ell## values (peaking at around ##\ell=1000## or so).
 
And how is it useful to determine the features of DE?
 
What do you mean useful?

Many people think dark energy is interesting, and understanding dark energy would give us deeper insight into the most fundamental physical laws. But I don't know what you mean by, "useful."
 
No, I'm just asking how what you wrote in your answer could be used to determine the presence of dark energy
 
astrolollo said:
No, I'm just asking how what you wrote in your answer could be used to determine the presence of dark energy
Ahhh, that makes more sense :)

Dark energy's effect on the CMB is pretty small. For the most part it impacts the low-##\ell## modes through the ISW effect. The effect of lensing on the CMB has very little to do with dark energy.

What you get from gravitational lensing that is useful for dark energy are measurements of the distribution of large scale structure. Measurements of the distribution of large scale structure provide a measurement of how the universe has expanded over time, which is currently the primary method for detecting dark energy (dark energy changes the rate of expansion depending upon its amount and properties).
 
"The effect of lensing on the CMB has very little to do with dark energy." Are you sure? Planck alone can put some constraints on DE only with lensing..
 
Not from lensing, I don't think. Or, at least, not directly. The main direct constraint comes from the ISW effect, which isn't a matter of lensing. As I mentioned, lensing can be used to measure large scale structure, and that can be used to measure dark energy through the rate of change of expansion.
 

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