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