DE Nature and Observational Probes

Dark Energy4:
DE Nature and Observational Probes
GUASA 2015
Octavio Valenzuela
IA-UNAM
Historia de la Energía Oscura
Dic. 1997: Contradicciones en nuestro entendimiento
del Universo!
Edad del Universo < a la de cúmulos globulares
- Poca estructura a pequeña escala
- Densidad promedio
-
Paradigma teórico motivado por simplicidad:
Universo plano, compuesto de materia (normal + oscura fría),
distribución de perturbaciones iniciales independiente de la escala.
El modelo tenía que cambiar o extenderse!
una de las hipótesis al menos-“plano,” “fría,” “invariante de escala,” o quizás “de materia.”
Fainter
High Redshift SN and Expansion History: Acceleration
Just recently
1.0
Found: Hubble,
Followed: Hubble
Dark Matter Dominated
Found: Ground,
Followed: Hubble
Dark Energy Dominated
Relative Brightness (Δm)
Constant acceleration
0.5
Acceleration/
Deceleration
0.0
Freely expanding
-0.5
Constant deceleration
Brighter
-1.0
0.0
present
0.5
1.0
Redshift z
1.5
2.0
past
Radiación de fondo proporciona una
regla estándar (Velocidad del sonido
en el plasma primordial)
400,000 año después del Big Bang
400,000 años luz. Horizonte acústico
depende cosmología, bariones, fotones
ΩTot = [θpeak(deg)]-1/2.
Observación: θpeak = 1o.
flat
El universo es plano:
positively
curved
negatively
curved
ΩTot = 1 .
[Miller et al.; de Bernardis et al; WMAP]
Concordance:
ΩΜ = 0.3,
ΩΛ = 0.7 .
¿What is the DE Nature?
•
Cosmological Constant?
•
Other?
•
How can we test it?
•
Stay tuned
Aceleración restringe la relación entre presión y densidad
del Universo es decir
la ecuación de estado
w=
/
w < -1/3 para que haya aceleración
¿Is w, constant? We must accurately measure the expansion history?
PPProbing DE via cosmology
• We “see” dark energy through its effects on the expansion of the
universe: • Three (3) main approaches – Standard candles • measure dL (integral of H-1) – Standard rulers • measure dA (integral of H-1) and H(z) – Growth of fluctuations. • Crucial for testing extra ! components vs modified gravity. Standard Ruler
• Suppose we had an object whose length we know as a function of
cosmic epoch. • By measuring the angle (⍬) subtended by this ruler (✗) as a function of
redshift we map out the angular diameter distance dA • By measuring the redshift interval (△z) associated with this distance we
map out the Hubble parameter H(z) Slide from Shirley Ho
pressure
vs
selfgravity
Oscillations!!
Non-Linear
Linear
It happens
only with baryons presence
Is this our Universe? Do you believe it?
DESI
Using Hubble Law
to map galaxy 3D
distribution
oh no!!Deformation
of BAO peak
DESI
El crecimiento de la estructura cósmica depende de la
naturaleza de la energía oscura
efecto de la expansión del universo
¿What is the DE Nature?
•
Cosmological Constant?
•
Other?
•
How can we test it?
1. Vacuum Energy (the Cosmological Constant)
What we know about dark energy:
smoothly distributed through space?
varies slowly (if at all) with time
ρ ≈ constant (w ≈ -1)
Dark energy could be exactly
constant through space and
time: vacuum energy (i.e.
the cosmological constant Λ).
(artist's impression
of vacuum energy)
Energy of empty space.
Minimum Energy Level of Fields
People sometimes pretend there is a difference
between a cosmological constant,
and a vacuum energy,
There’s not; just set
.
Well, an intrinsic geometrical space time feature?
Problem One:
Why is the vacuum
energy so small?
We know that virtual particles
couple to photons (e.g. Lamb
shift, Casimir forces); why not to gravity?
e-
e-
photon
graviton
e+
Naively: ρvac = ∞, or at least ρvac = EPl/LPl3 = 10120 ρvac(obs).
e+
Problem Two:
Why is the vacuum
energy important now?
You
are
here
We seem to be living in a
special time. Copernicus
would not be pleased.
Could we just be lucky?
The Gravitational Physics Data Book:
Newton's constant:
G = (6.67 ± 0.01) x 10-8 cm3 g-1 sec-2
Cosmological constant:
Λ = (1.2 ± 0.2) x 10-55 cm-2
If we set h = c = 1, we can write
G = EPlanck-2 and ρvac = Evac4 , and
EPlanck = 1027 eV ,
Different by 1030.
Εvac = 10-3 eV .
Supersymmetry can squelch the vacuum energy; unfortunately,
in the real world it must be broken at ESUSY ~ 1012 eV.
Typically we would then expect
which is off by 1015. But if instead we were able to predict
it would agree with experiment. (All we need is a theory
that predicts this relation!)
1027 eV
1012 eV
10-3 eV
energy
EPlanck
Esusy
Evac
Is environmental selection at work?
String theory has extra
dimensions, with a vast
“landscape” of ways to hide
them. Perhaps 10500 or more.
The “constants of nature”
we observe depend on the
shape and size of the
compact manifold.
Everything changes from
one compactification to
the next, including the
value of the vacuum energy.
[Bousso & Polchinski; Kachru et al.]
Maybe each compactification actually exists somewhere.
Regions outside our observable universe, where the laws
of physics and constants of nature appear to be different.
In that case, vacuum
energy would be like
the weather; not a
fundamental parameter,
but something that
depends on where you
are in the universe.
Assume Ensemble of possibilities and estimate
Therefore (so the reasoning goes), it's hardly surprising
that we find such a tiny value of the vacuum energy –
regions where it is large and simply inhospitable.
[Weinberg]
2. Dynamical Dark Energy (Quintessence: Scalar Field)
Dark energy doesn’t vary quickly, but maybe slowly.
V(φ)
kinetic
energy
gradient
energy
potential
energy
φ
[Wetterich; Peebles & Ratra;
Caldwell, Dave & Steinhardt; etc.]
This is an observationally interesting possibility.
Might be relevant to the cosmological constant problem
or the coincidence scandal -- somehow.
torsion-balance experiment
Coupling to a low-mass (long-range)
field implies a fifth force of nature,
which can be searched for in
laboratory experiments.
Also: gradual evolution
of physical constants as the
field evolves.
[Adelberger et al.]
Emission Lines separation at
different Redshift
Limit: couplings must be
suppressed by ~ 105 MP.
[Webb et al.]
Both fine-tunings -- mass and interactions -- can be
addressed in one fell swoop, by imagining a
slightly broken symmetry
V(φ)
φ
[Frieman et al; Carroll]
Then the quintessence
is a pseudo-NambuGoldstone boson,
with a cosine potential
and naturally small
mass and interactions.
But one interaction is allowed -- a parity-violating
term of the form
, coupling quintessence to
the electromagnetic fields.
This interaction produces cosmological birefringence:
polarization vectors rotate as they travel through
the evolving scalar field.
WMAP 5-year data:
Radio galaxies also provide
interesting constraints.
.
So:
1.
2.
3.
A cosmological constant fits the data, at the
expense of a dramatic fine-tuning.
Dynamical models introduce new fine-tunings,
in the form of the small mass and couplings of
the new scalar field.
Dynamical models have not yet shed any light on
the cosmological constant problem or the
coincidence scandal.
3. Modified Gravity
Simplest possibility: replace
with
[Carroll, Duvvuri,
Trodden & Turner 2003]
The vacuum in this theory is not flat
space, but an accelerating universe!
But: the modified action brings a
new tachyonic scalar degree of
freedom to life. A scalar-tensor theory of gravity.
Scalar-Tensor Gravity
Introduce a scalar field φ (x) that determines the
I strength of gravity. Einstein's equation
n
t
is replaced by
variable “Newton's constant”
extra energy-momentum from
The new field φ (x) is an extra degree of freedom;
an independently-propagating scalar particle.
φ
The new scalar φ doesn’t
interact directly with
matter, because we say
so. But it does influence
the metric.
A natural value for the
Brans-Dicke parameter
ω would be
ω~1,
where ω = 1 is GR.
[Chiba 2003]
Experiments imply
ω > 40,000 .
Cassini
Loophole: the Chameleon Effect.
V
Curvature contributes to
the effective potential
for φ. With the right
bare potential, the field
can be pinned (with
large mass) in dense
regions, e.g. the galaxy.
Deviations from GR can be
hidden on sub-galactic scales. How?
n
V0
[Khoury & Weltman;
Hu & Sawicki]
Dvali, Gabadadze, & Porrati (DGP) gravity: an infinite
extra dimension, with gravity stronger in the bulk;
5-d kicks in at large distances.
4-d gravity
5-d GR
5-d gravity suppressed by rc ~ H0-1
gravity term
rc 5-d
~ H0-1
suppressed by rc ~ H0-1
r* = (rS rc2)1/3
crossover
Needs
r = 2GM
Large Volume
Surveys
4-d GR
DESI, LSST, EUCLID?
S
[Dvali, Gabadadze & Porrati 2000]
Self-acceleration in DGP cosmology
The DGP version of the Friedmann equation is (naturally):
This exhibits self-acceleration: for ρ = 0, there is a
de Sitter solution with H = 1/rc = constant. However:
The acceleration is somewhat mild; think weff ~ -0.7. Inconsistent with present data at about 5σ.
Fluctuations of the brane have negative energies
(ghosts). Hard to fix this problem.
So:We may expect GR to be modified:
1.
on short scales,
It may be
This observations
set important
constraints
El centro de gravedad no coincide con la materia luminosa
So:We may expect GR to be modified:
1.
on short scales,
2.
on long scales?, difficult
3.
4.
5.
but it could happen, not discarded.
f(R) gravity can fit the data, but only through
various fine-tunings (over and above the cosmological constant and coincidence problems)
and the chameleon mechanism.
DGP gravity doesn’t really fit the data , and has
issues with negative-energy ghosts.
•
Cosmic Acceleration may indicate:
•
Vacuum Energy
•
New Field
•
New Gravity
•
How to disentangle? Expansion history, structure
growth?
•
Does it work?
RSD
¿Qué Hacer?
#
#
#
#
Medir con gran precisión:
La historia de expansión del Universo, se acelera
siempre igual o cambia esta aceleración? (BAO)
Lentes Gravitacionales
El crecimiento de la estructura: como se agregan a la
estructura las galaxias (Redshift space distortion)
Conclusiones
•
La naturaleza de la Energía Oscura es un reto
•
Las mediciones recientes sugieren que es consistente con una
constante pero grandes inconsistencias sugieren más elementos
•
La opción de una nueva teoría de gravedad suena atractiva pero es
mucho más compleja de lo que se esperaba, no existe una opción
completa, aunque es un reto muy grande
•
Un nuevo campo podría representar a esta energía oscura, pero
puede afectar fuertemente el crecimiento de estructura.
•
El la siguiente década los nuevos catastros de galaxias definirán
fuertes restricciones a las explicaciones