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TYPE Ia SUPERNOVAE, EVOLUTION, AND THE COSMOLOGICAL CONSTANT

机译：Ia型超新星，演化和宇宙常数

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We explore the possible role of evolution in the analysis of data on Type Ia supernovae (SNe Ia) at cosmological distances. First, using a variety of simple sleuthing techniques, we find evidence that the properties of the high- and low-redshift SNe Ia observed so far differ from one another. Next, we examine the effects of allowing for an uncertain amount of evolution in the analysis, using two simple phenomenological models for evolution and prior probabilities that express a preference for no evolution but allow it to be present. One model shifts the magnitudes of the high-redshift SNe Ia relative to the low-redshift SNe Ia by a fixed amount. A second, more realistic, model introduces a continuous magn- tude shift of the form δm(z) = βln(1 + z) to the SNe Ia sample. The result is that cosmological models and evolution are highly degenerate with one another, so that the incorporation of even very simple models for evolution makes it virtually impossible to pin down the values of Ω_m and Ω_Λ the density parameters for nonrelativistic matter and for the cosmological constant, respectively. The Hubble con- stant, H_o, is unaffected by evolution. We evaluate the Bayes factor for models with evolution versus models without evolution, which, if one has no prior predilection for or against evolution, is the odds ratio for these two classes of models. The resulting values are always of order 1, in spite of the fact that the models that include evolution have additional parameters, thus, the data alone cannot discriminate between the two possibilities. Simulations show that simply acquiring more data of the same type as are available now will not alleviate the difficulty of separating evolution from cosmology in the analysis. What is needed is a better physical understanding of the SN Ia process, and the connections among the maximum luminosity, rate of decline, spectra, and initial conditions, so that physical models for evolu- tion may be constructed, and confronted with the data. Moreover, we show that if SNe Ia evolve with time, but evolution is neglected in analyzing data, then, given enough SNe Ia, the analysis hones in on values of Ω_M and Ω_Λ that are incorrect. Using Bayesian methods, we show that the probability that the cosmological constant is nonzero (rather than zero) is unchanged by the SNe Ia data when one accounts for the possibility of evolution, provided that we do not discriminate among open, closed, and flat cos- mologies a priori. The case for nonzero cosmological constant is stronger if the universe is presumed to be flat but still depends sensitively on the degree to which the peak luminosities of SNe Ia evolve as a function of redshift.
机译：我们探索在宇宙距离上Ia型超新星（SNe Ia）数据分析中进化的可能作用。首先，我们使用各种简单的侦查技术发现了迄今为止观察到的高红移和低红移SNe Ia的特性彼此不同的证据。接下来，我们使用两个简单的现象学模型对演化和先验概率进行分析，以分析中不确定数量的演化所产生的影响，这些先验概率表达了对不演化但允许存在的偏好。一种模型将高红移SNe Ia的幅度相对于低红移SNe Ia的幅度偏移固定量。第二个更现实的模型向SNe Ia样本引入了δm（z）=βln（1 + z）形式的连续大位移。结果是，宇宙学模型和演化彼此高度退化，因此，即使是非常简单的演化模型的结合，也几乎不可能确定非相对论物质和宇宙常数的Ω_m和Ω_Λ的密度参数， 分别。哈勃常数H_o不受进化的影响。我们评估具有演化模型的模型与没有演化模型的模型的贝叶斯因子，如果没有对演化的先验偏爱，则为这两类模型的比值比。尽管包含演化的模型具有附加参数，但结果值始终为1，因此，仅凭数据无法区分这两种可能性。模拟表明，仅获取与现在可用的相同类型的更多数据并不会减轻分析中将演化与宇宙学分离的难度。需要的是对SN Ia过程的更好的物理理解，以及最大光度，下降率，光谱和初始条件之间的联系，以便可以构造用于演化的物理模型并与数据面对。此外，我们表明，如果SNe Ia随时间变化，但在分析数据时忽略了进化，那么，如果有足够的SNe Ia，则分析会错误地考虑Ω_M和Ω_Λ的值。使用贝叶斯方法，我们表明，当我们考虑演化的可能性时，只要我们不区分开放，封闭和平坦cos，那么SNe Ia数据就可以证明宇宙常数为非零（而不是零）的概率不变。 -先验学。如果假定宇宙是平坦的，则非零宇宙学常数的情况会更强，但仍敏感地取决于SNe Ia的峰值光度随红移而演化的程度。

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