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Register a new userThe Phantom Bounce: A New Oscillating Cosmology
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An oscillating universe cycles through a series of expansions and contractions. We propose a model in which ``phantom energy with $p < -rho$ grows rapidly and dominates the late-time expanding phase. The universes energy density is so large that the effects of quantum gravity are important at both the beginning and the end of each expansion (or contraction). The bounce can be caused by high energy modifications to the Friedmann equation, which make the cosmology nonsingular. The classic black hole overproduction of oscillating universes is resolved due to their destruction by the phantom energy.
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The phantom brane has several important distinctive features: (i) Its equation of state is phantom-like, but there is no future `big rip singularity, (ii) the effective cosmological constant on the brane is dynamically screened, because of which the expansion rate is {em smaller} than that in $Lambda$CDM at high redshifts. In this paper, we constrain the Phantom braneworld using distance measures such as Type Ia supernovae (SNeIa), Baryon Acoustic Oscillations (BAO), and the compressed Cosmic Microwave Background (CMB) data. We find that the simplest braneworld models provide a good fit to the data. For instance, BAO +SNeIa data can be accommodated by the braneworld for a large region in parameter space $0 < Omega_l < 0.3$ at $1sigma$. The Hubble parameter can be as high as $H_0 < 78$ km/s/Mpc, and the effective equation of state at present can show phantom-like behaviour with $w_0 < -1.2$ at $1sigma$. We note a correlation between $H_0$ and $w_0$, with higher values of $H_0$ leading to a lower, and more phantom-like, value of $w_0$. Inclusion of CMB data provides tighter constraints $Omega_l < 0.1$. (Here $Omega_l$ encodes the ratio of the five and four dimensional Planck mass.) The Hubble parameter in this case is more tightly constrained to $H_0 < 71$ km/s/Mpc, and the effective equation of state to $w_0 < -1.1$. Interestingly, we find that the universe is allowed be closed or open, with $-0.5 < Omega_{kappa} < 0.5$, even on including the compressed CMB data. There appears to be some tension in the low and high $z$ BAO data which may either be resolved by future data, or act as a pointer to interesting new cosmology.
We show that the f(T) gravitational paradigm, in which gravity is described by an arbitrary function of the torsion scalar, can provide a mechanism for realizing bouncing cosmologies, thereby avoiding the Big Bang singularity. After constructing the simplest version of an f(T) matter bounce, we investigate the scalar and tensor modes of cosmological perturbations. Our results show that metric perturbations in the scalar sector lead to a background-dependent sound speed, which is a distinguishable feature from Einstein gravity. Additionally, we obtain a scale-invariant primordial power spectrum, which is consistent with cosmological observations, but suffers from the problem of a large tensor-to-scalar ratio. However, this can be avoided by introducing extra fields, such as a matter bounce curvaton.
We investigate whether successful Gravitational Leptogenesis can take place during an Ekpyrotic contraction phase. Two possible paths by which this can occur are coupling the Ekpyrotic scalar to a gravitational Chern-Simons term, or to a $ U(1) $ gauge field Chern-Simons term. These couplings lead to the production of chiral gravitational waves, which generate a lepton number asymmetry through the gravitational-lepton number anomaly. This lepton asymmetry is subsequently reprocessed by equilibrium sphaleron processes to produce a baryon asymmetry. We find successful Gravitational Leptogenesis to be possible in Ekpyrotic bounce cosmologies through both of these mechanisms.
We study a nonsingular bounce inflation model, which can drive the early universe from a contracting phase, bounce into an ordinary inflationary phase, followed by the reheating process. Besides the bounce that avoided the Big-Bang singularity which appears in the standard cosmological scenario, we make use of the Horndesky theory and design the kinetic and potential forms of the lagrangian, so that neither of the two big problems in bouncing cosmology, namely the ghost and the anisotropy problems, will appear. The cosmological perturbations can be generated either in the contracting phase or in the inflationary phase, where in the latter the power spectrum will be scale-invariant and fit the observational data, while in the former the perturbations will have nontrivial features that will be tested by the large scale structure experiments. We also fit our model to the CMB TT power spectrum.
Theories with compact extra dimensions are sometimes unstable to decay into a bubble of nothing -- an instability resulting in the destruction of spacetime. We investigate the existence of these bubbles in theories where the moduli fields that set the size of the extra dimensions are stabilized at a positive vacuum energy -- a necessary ingredient of any theory that aspires to describe the real world. Using bottom-up methods, and focusing on a five-dimensional toy model, we show that four-dimensional de Sitter vacua admit bubbles of nothing for a wide class of stabilizing potentials. We show that, unlike ordinary Coleman-De Luccia tunneling, the corresponding decay rate remains non-zero in the limit of vanishing vacuum energy. Potential implications include a lower bound on the size of compactified dimensions.
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