Kinematic cosmic dipole from a large sample of strong lenses

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: We're looking at the paper: "Kinematic cosmic dipole from a large sample of strong lenses".

Jocelyn: Here's the gist of the paper. The gist Measurements of strong gravitational lenses allow for an independent estimation of the kinematic cosmic dipole, which can discriminate between values inferred from the CMB and those from high-redshift source number counts How it works The proposed method estimates the observer's peculiar velocity by using strong gravitational lenses with well-measured Einstein radii to estimate the kinematic cosmic dipole through relativistic aberration of the Einstein angle induced by the observer’s motion The observable is derived purely from special relativity, where a non-comoving observer perceives relativistic aberration that induces a direction-dependent modulation of the observed Einstein radius The boosted Einstein radius in direction eˆ′ 2 is given by θ′E = θE 1 − vo c cos θ′cm = θE 1 − vo c cos θ′cm (2.5) Measurement of the observer velocity from lensing alone The measurement relies on fitting a monopole and a dipole to the catalog of azimuthally averaged Einstein radii measurements with associated galactic coordinates to maximize the likelihood function The likelihood definition involves maximizing L1, which fits a displacement of the mean Einstein–radius distribution as a function of position on the sky When relying on lensing information alone, the Euclid lens sample is unlikely to provide strong discriminatory power between the CMB-inferred dipole amplitude and larger values suggested by source number count analyzes, with constraints not sufficiently precise to exclude the CMB-inferred value beyond the ∼ 1.2σ level Combining lensing and stellar kinematics The precision of the measurement can be significantly improved by incorporating additional lens kinematic information, either from spectroscopic measurements or inferred via the Fundamental Plane (FP) relation The fractional measurement error on σv translates to θE according to δσv/σv = 1/2 δθE/θE (4.2) Spectroscopic velocity dispersion provides an estimate of the mass of each lensing galaxy, which largely reduces the noise term in the likelihood The FP relation relates the effective radius Re, central velocity dispersion σFP, and mean surface brightness Ie Euclid Forecast and discrimination power The combination of lensing information with kinematic data shows significant gains in constraining power; for the “400k” sample, the discriminating power reaches ∼ 2σ In a perfect scenario with all lenses having spectroscopic redshifts for both the lens and the source, this probe could make a decisive contribution to this debate, with a discriminating power reaching ∼ 5σ Strong-lensing measurements can reach a sufficient level of precision to discriminate between the CMB-inferred and source number-count values of vo, provided that adequate spectroscopic follow-up data are available for a large fraction of the Euclid strong-lens sample The method is mostly insensitive to the lens selection function, provided it is spatially uniform over the survey footprint Similarly, any bias in the lens mass modeling or in the dynamical modeling of the lenses can be self-calibrated from the data and does not affect the measurement of vo, as long as these biases are constant across the sky The recent discovery of hundreds of new strong-lens systems in the Euclid Q1 data highlights the potential to increase the current sample of known lenses by two orders of magnitude with the full Euclid survey Page 4 --- The gist Measurements of strong gravitational lenses allow for an independent estimation of the kinematic cosmic dipole, which can discriminate between values inferred from the CMB and those from high-redshift source number counts Measurement of the observer velocity from lensing alone The measurement relies on fitting a monopole and a dipole to the catalog of azimuthally averaged Einstein radii measurements with associated galactic coordinates to maximize the likelihood function When relying on lensing information alone, the Euclid lens sample is unlikely to provide strong discriminatory power between the CMB-inferred dipole amplitude and larger values suggested by source number count analyzes, with constraints not sufficiently precise to exclude the CMB-inferred value beyond the ∼ 1.2σ level Strong-lensing measurements can reach a sufficient level of precision to discriminate between the CMB-inferred and source number-count values of vo, provided that adequate spectroscopic follow-up data are available for a large fraction of the Euclid strong-lens sample The method is mostly insensitive to the lens selection function, provided it is spatially uniform over the survey footprint Similarly, any bias in the lens mass modeling or in the dynamical modeling of the lenses can be self-calibrated from the data and does not affect the measurement of vo, as long as these biases are constant across the sky Page 7 --- The gist Measurements of strong gravitational lenses allow for an independent estimation of the kinematic cosmic dipole, which can discriminate between values inferred from the CMB and those from high-redshift source number counts How it works The proposed method estimates the observer's peculiar velocity by using strong gravitational lenses with well-measured Einstein radii to estimate the kinematic cosmic dipole through relativistic aberration of the Einstein angle induced by the observer’s motion The observable is derived purely from special relativity, where a non-comoving observer perceives relativistic aberration that induces a direction-dependent modulation of the observed Einstein radius Combining lensing and stellar kinematics The precision of the measurement can be significantly improved by incorporating additional lens kinematic information, either from spectroscopic measurements or inferred via the Fundamental Plane (FP) relation The fractional measurement error on σv translates to θE according to δσv/σv = 1/2 δθE/θE (4.

Paper summary: Vera: So we're looking at this paper called "Kinematic cosmic dipole from a large sample of strong lenses." Basically, it tackles that tension we see between what the Cosmic Microwave Background tells us about our motion and what we get from counting distant sources.

Jocelyn: Right, it claims they can use strong gravitational lenses with well-measured Einstein radii to estimate that kinematic cosmic dipole independently. The idea is to look at how the observer's motion causes a relativistic aberration of the Einstein angle when we measure those lens radii.

Subrahmanyan: So the core claim is that this effect could be detected just by measuring the Einstein radius in wide, high-resolution imaging surveys like Euclid. But they also point out that using just those Einstein radii measurements, without knowing redshift or lens galaxy mass information, it's not precise enough to really separate the CMB value of our peculiar velocity from the values suggested by source number counts at a high statistical significance.

Vera: That makes sense because we need a better tool than just the raw Einstein radius measurement alone to get a solid result. They show that if you include a large sample of lenses with kinematic information, like from the Fundamental Plane relation or ideally through spectroscopic velocity dispersion measurements, the noise drops significantly and the constraining power gets much stronger.

Jocelyn: It sounds like they're saying that getting those extra kinematic details from spectroscopy or by using things like the Fundamental Plane drastically reduces the error on our measurement of observer velocity. They even show that with a realistic sample of strong lenses detected by Euclid, combined with spectroscopic measurements from things like 4MOST or DESI, you can discriminate between the CMB and source-number-counts-inferred values at about five sigma <ref:2603.11152#pg1>.

Subrahmanyan: That's a big jump from what they could do just with lensing data alone, showing that the combination of lenses and kinematics is where the real power comes from for this specific problem. What I find interesting is how they tackle those selection effects, noting that their method is only weakly sensitive to strong-lensing selection effects, with biases estimated to be well below one percent.

Vera: So, while it's not perfectly immune to bias, the authors suggest that if the survey footprint is spatially uniform, then this technique holds up well for measuring our peculiar velocity. But they are clear about what they aren't doing; the method doesn't account for every possible bias in how we model lens mass or dynamical modeling of those lenses unless those biases are constant across the sky.

Jocelyn: It seems like the main thing here is showing a path forward: if we can get that spectroscopic follow-up data for a large fraction of Euclid's strong lenses, this method becomes a viable way to settle this debate about the true value of our peculiar velocity.

Subrahmanyan: When you look at the overall picture, this paper points toward using independent probes, like strong lensing kinematics, to test cosmological parameters derived from the CMB against those derived from galaxy counts. It suggests that getting good kinematic data on lenses is a crucial missing piece in understanding how structure forms and moves around.

Vera: And it opens up a clear path for future work: getting those spectroscopic velocity dispersion measurements from 4MOST or DESI for these Euclid strong lenses is the next big step to really making this method robust <ref:2603.11152#pg1>.

Jocelyn: So, to wrap up, the paper "Kinematic cosmic dipole from a large sample of strong lenses" shows how using relativistic aberration on Einstein radii can estimate our peculiar velocity. It proves that by adding kinematic data like velocity dispersion or the Fundamental Plane relation, we can get a high-precision measurement that could help us distinguish between different estimates of our motion.

Subrahmanyan: Exactly. The implication is that this specific lensing method offers a new way to test cosmological models against observational data on structure formation and source counts. It’s about using geometry to probe dynamics.

Vera: So, the authors are setting up a clear test: if we have high-quality kinematic data from Euclid lenses, we can get a five sigma measurement that separates the CMB value of v o from the source number count values <ref:2603.11152#pg2>.

Jocelyn: That's it. It gives us a concrete way to move past the tension by using this new geometric approach combined with good kinematics.

Conclusion: Vera: So, the title of this paper is "Kinematic cosmic dipole from a large sample of strong lenses," and they are looking at how we can use these lenses to measure our velocity.

Jocelyn: Right, so they're using those Einstein radii—which are measured by the size of the warped light around a lens—to estimate our motion because our movement causes a relativistic shift in how those radii look from different directions.

Subrahmanyan: The core idea is that this effect, this relativistic aberration, depends on the observer's velocity v o, and they want to see if they can pin down the magnitude of that velocity.

Vera: They show that using just these Einstein radii measurements from a big survey like Euclid isn't enough to clearly separate the CMB value from other estimates of our motion, which is a bit disappointing.

Jocelyn: It seems like they've hit a hurdle where without some extra data, the constraints are not precise enough to rule out the CMB value beyond about one point two sigma.

Subrahmanyan: That makes sense because you need more information than just the geometry of the lens itself to get a strong constraint on cosmology.

Vera: But they do show that if you add in some kinematic info, like using the Fundamental Plane relation or getting spectroscopic velocity dispersion measurements, things get much better.

Jocelyn: Yeah, they show that when you combine those lensing measurements with real velocity data from things like 4MOST or DESI lenses, you can reach about five sigma discrimination between the CMB and source-number-count values <ref:2603.11152#pg1>.

Subrahmanyan: That's a significant improvement because it shows this method can be very powerful if we get that spectroscopic follow-up data for a large part of the Euclid lens sample.

Vera: The authors also noted that this technique isn't really sensitive to selection effects from how we pick which lenses to look at, as long as the survey footprint is fairly uniform across the sky.

Jocelyn: That’s good because it means we don't have to worry too much about biases in just picking the right objects for our measurements.

Subrahmanyan: But they also pointed out that any bias in how you model the mass of each lens, or how you model their dynamics, won't affect the measurement of v o as long as those biases are constant across the sky.

Vera: So they're essentially saying that if we can get good kinematics for a large sample, this method becomes a very independent way to test our motion.

Jocelyn: It opens up a lot of avenues for how we look at the relationship between structure formation and cosmology.

Subrahmanyan: And it sets up the next big thing, which is getting that spectroscopic follow-up data to really see if this five sigma discrimination holds up in reality.