A screened time-drag scalar field: percent-level late-time growth and the cosmic radio dipole excess

A sc eened ime-d ag scala ield: pe cen -le el la e- ime g ow h
and he cosmic adio dipole excess
Paul Cooney1
1Independen esea che , Innis il, On a io, Canada∗
(Da ed: No embe 22, 2025)
We p esen a minimal, ghos - ee, g adien -s able scala – enso heo y in which a single scala ield
τwi h a densi y-dependen kine ic coe icien Z(ρm) p oduces ∼1% modi ica ions o he g ow h
a e a z≲2 while p ese ing he exac ΛCDM backg ound expansion. Kine ic sc eening supp esses
i h o ces by mo e han 14 o de s o magni ude locally and by >1024 a ecombina ion. Linea
pe u ba ions yield a ime-dependen e ec i e g a i a ional s eng h Ge (a) = G[1+αe (a)] wi h
αe (a)≃(α/2) Ωm(a)/[Ωm(a) + ΩΛ] (α∼0.02). La ge-scale g adien s in he backg ound alue
o ˙τinduce a non-kinema ic con ibu ion o numbe -coun dipoles o o de Dτ∼0.01–0.02 ha is
au oma ically aligned wi h he CMB dipole, peaks a z∼1, and anishes a bo h low and high
edshi . The model is consis en wi h all cu en cons ain s om BBN, CMB, Sola Sys em, and
g a i a ional-wa e obse a ions, and makes sha p p edic ions o upcoming DESI, Euclid, LSST
g ow h measu emen s and SKA/LSST edshi -binned dipoles.
I. INTRODUCTION
Al hough ΛCDM emains ema kably success ul, wo
mild la e- ime anomalies pe sis :
(i) a ∼2–3σp e e ence o enhanced clus e ing ampli-
ude in low- edshi p obes [1,2], and
(ii) a ac o ∼2–4 excess in he local adio and mid-
in a ed sou ce-coun dipole ampli ude ela i e o
he kinema ic expec a ion [3,4].
We in oduce a single-pa ame e , s ongly sc eened
scala – enso ex ension ha simul aneously accoun s o
bo h anomalies.
II. ACTION AND SCREENING
The ac ion is
S=Zd4x√−gM2
Pl
2R+1
2Z(ρm)gµν ∂µτ∂ντ−ρΛ+Lm,
(1)
whe e he densi y-dependen kine ic coe icien is
Z(ρm)=αρm
1+(ρm/ρ∗)4, ρ∗≃5×10−27 h2g cm−3.
(2)
Ma e couples minimally o he Jo dan- ame me ic
gµν .
∗paul.co[email p o ec ed] o.ca; Zenodo DOI: 10.5281/zen-
odo.17678527
III. BACKGROUND ATTRACTOR
On FLRW backg ounds he scala obeys he la e- ime
a ac o
˙τ2=ρm(a)
ρm(a)+ρΛ
,(3)
which ensu es ha i s kine ic ene gy densi y ρkin =
(α/2)ρ2
m/(ρm+ρΛ) emains much smalle han bo h ρm
and ρΛ o α≲0.03. The expansion his o y is he e o e
indis inguishable om ΛCDM.
IV. LINEAR PERTURBATIONS AND Ge (a)
In he quasi-s a ic, sub-ho izon egime he modi ied
Poisson equa ion eads
k2Φ=−4πGe (a)a2ρmδm,(4)
wi h he ime-dependen e ec i e g a i a ional s eng h
Ge (a) = G1 + α
2
Ωm(a)
Ωm(a)+ΩΛ.(5)
This yields a ∼1% enhancemen o he g ow h a e a
la e imes (z≲2).
V. THEORETICAL CONSISTENCY AND
CONSTRAINTS
A. S abili y
The heo y admi s a k-essence ep esen a ion (Ap-
pendix A)
P=αρm+ρΛ
ρm
X−ρΛ,(6)
wi h PX>0, PXX = 0, and c2
s= 1, gua an eeing ab-
sence o ghos s and g adien ins abili ies.
2
0.0 0.5 1.0 1.5 2.0
Redshi
z
10
5
0
5
10
(
8)/(
8)Planck
(%)
= 0.02
model
BOSS DR12
FIG. 1. Pe cen age enhancemen o σ8(z) ela i e o Planck
ΛCDM o α= 0.02. Da a poin s: DESI 2024 (blue),
BOSS/eBOSS (o ange), and KiDS+DES (g een).
B. Sc eening and i h o ces
In o e dense egions Z∝ρ−3
m, so scala couplings a e
supp essed by ≳10−14 in he Sola Sys em and by mo e
han 1024 a ecombina ion. All Sola Sys em, BBN,
CMB, g a i a ional-wa e speed, and PPN cons ain s a e
he e o e easily sa is ied.
VI. NUMBER-COUNT DIPOLE
CONTRIBUTION
La ge-scale densi y g adien s ha sou ce ou peculia
eloci y also induce di ec ional a ia ion in he back-
g ound alue o ˙τ. Using he ull ela i is ic numbe -
coun o malism [5,6], he non-kinema ic dipole con i-
bu ion is
Dτ(z)≃0.018 α
0.02ΩΛ
Ωm(z)+ΩΛ⟨2+dln n/d ln L⟩.
(7)
Fo ypical adio and mid-in a ed luminosi y unc ions
wi h ⟨2+dln n/d ln L⟩≃3–5, he peak ampli ude a
z≃1 lies in he ange 0.016–0.020, in good ag eemen
wi h obse a ions.
VII. CONCLUSIONS
A single dimensionless coupling α≃0.02 in a s ongly
sc eened scala – enso heo y na u ally p oduces:
•a∼1% la e- ime enhancemen o he g ow h a e,
and
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Median edshi
z
0.0
0.5
1.0
1.5
2.0
2.5
Dipole ampli ude (%)
Kinema ic only
Kinema ic + (
= 0.02
)
NVSS
Ca WISE
RACS/EMU
FIG. 2. To al p edic ed dipole ampli ude (kinema ic plus
τ-induced) as a unc ion o median edshi o α= 0.02.
Da a poin s om NVSS, Ca WISE, and SKA p ecu so s a e
o e laid.
•a non-kinema ic con ibu ion o he cosmic
adio/mid-in a ed dipole o he obse ed magni-
ude, di ec ion, and edshi dependence.
Bo h signa u es will be es ed a pe cen -le el p eci-
sion by o hcoming DESI, Euclid, LSST g ow h mea-
su emen s and by SKA and LSST edshi -binned dipole
su eys.
Appendix A: k-essence comple ion
The Lag angian P=α(ρm+ρΛ)X/ρm−ρΛexac ly
ep oduces he a ac o solu ion (Eq. 3) wi h luminal
sound speed c2
s= 1 and no ghos o g adien ins abili ies.
Appendix B: G a i a ional slip
In he quasi-s a ic, sub-ho izon limi he scala induces
no aniso opic s ess, so
Ψ = Φ.(B1)
The g a i a ional slip pa ame e he e o e sa is ies η−
1≲10−3.
3
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