不同天文观测对宇宙学模型的限制

魏俊杰

(中国科学院紫金山天文台 南京 210008)

博士学位论文摘要选登

不同天文观测对宇宙学模型的限制

魏俊杰†

(中国科学院紫金山天文台 南京 210008)

1998年,科学家们通过对Ia型超新星的观测研究,发现了宇宙在加速膨胀,揭示了暗能量的存在.该发现使得宇宙学成为当今物理学的研究热点.宇宙学是一门高度依赖于观测事实的学科,利用各类天文观测数据来限制不同的宇宙学理论模型是现代宇宙学的一项重要研究工作.

本论文的主要研究内容是利用不同的天文观测来限制宇宙学.这些天文观测包括Ia型超新星、超亮的Ic型超新星、伽玛射线暴、强引力透镜、星系团的角直径距离以及星系年龄.本文第1章简要回顾了宇宙学的研究背景,介绍了各种宇宙学模型,并详细总结了不同天文观测数据在宇宙学方面的研究进展.

第2章全面介绍了我们在超新星宇宙学方面的研究.Ia型超新星作为标准烛光的主要困难是,我们必须对表征超新星光度的3–4个“讨厌”参数连同宇宙膨胀模型的参量同时做拟合.对于拟合方法,我们认为应该采用最大似然估计法来拟合参数里含有未知内禀误差的模型.然而,传统的拟合方法是通过调节内禀误差的大小使得约化的χ2等于1,进而求出内禀误差的值.这种方法并没有考虑参数之间可能存在的方差.我们利用超新星信息巡天样本里的252颗超新星比较标准的ΛCDM模型和Rh=ct模型,发现每个模型都能够很好地拟合超新星数据.另外,现有观测证据已表明超亮的Ic型超新星也可作为宇宙学探针,它们甚至可以比Ia型超新星测量红移更远(z≫2)的宇宙.然而,目前可用的Ic型超新星样本还是很少.我们的模拟表明如果将来观测样本不断增多的话,超亮的Ic型超新星有望成为高效的宇宙学探针,它们可对宇宙学参数做出精确限制,特别是暗能量的状态方程参数wde.

第3章我们则专注于伽玛暴宇宙学的研究.首先,我们利用伽玛暴的光度关系来构造高红移处的哈勃图.目前的Ia型超新星观测只能测量低红移的宇宙,我们需要用高红移的伽玛暴来延伸哈勃图.其次,我们利用伽玛暴来限制高红移的恒星形成率.我们利用最新的Sw ift观测证实了高红移处的伽玛暴爆发率仍然超出恒星形成率的预期,伽玛暴爆发率与恒星形成率之间的不一致可以用宇宙金属丰度演化来解释.假定伽玛暴爆发率与恒星形成率和金属丰度演化有关,我们发现伽玛暴数据对高红移恒星形成率的斜率的限制为−2.41.此外,只有暗物质晕结构中的重子气体密度足够高时,重子气体才容易冷却并形成恒星.而且重子气体密度应该是正比于暗物质密度,因此恒星只能在大于一定质量的暗晕结构中形成.我们可用最小的暗晕质量Mmin来表征恒星开始形成.Mmin在恒星形成过程中起到了关键性作用.在68%的置信度区间内,伽玛暴数据对Mmin的限制为Mmin＜1012.5M⊙.

第4章我们收集了69个强引力透镜系统的观测数据.强引力透镜在宇宙学研究中所发挥的作用日渐凸显,我们仔细描述了如何用这类数据来限制宇宙学参数.尽管强引力透镜数据的观测精度还没有好到可以衡量哪个宇宙学模型更优,但是我们发现标准的ΛCDM模型和Rh=ct模型都可以很好地解释目前这些观测数据.

在第5和第6章中,我们分别利用星系团的角直径距离数据和32个古老星系的年龄测量来检验和比较标准的ΛCDM模型和Rh=ct模型,我们发现每个模型都能够很好地解释这两组数据.然而,这两个模型各自所含的自由参量个数不同,因此我们需要借助模型选择标准来判断哪个宇宙学模型的拟合优度更高.Akaike、Ku llback和Bayes信息选择标准表明,相比ΛCDM模型,这些观测数据更加倾向于支持Rh=ct模型,似然概率分别约为70%,75%和80%.

最后,第7章是作者在完成了以上研究工作之后对现阶段宇宙学研究中所存在问题的一点思考,以及对未来宇宙学研究前景的一些展望.

with the observations of Type Ia supernovae(SNe Ia),scientists discovered that the Universe is experiencing an accelerated expansion,and then revealed the existence of dark energy in 1998.Since the am azing discovery,cosm ology has became a hot topic in the physical research field.Cosm ology is a sub ject that strongly depends on the astronom ical observations.Therefore,constraining different cosmologicalmodels with all kinds of observations is one of themost important research works in themodern cosmology.

The goal of this thesis is to investigate cosm ology using the latest observations.The observations include SNe Ia,Type Ic Super Lum inous supernovae(SLSN Ic),Gamma-ray bursts(GRBs),angular diameter distanceofgalaxy cluster,strong gravitational lensing,and agem easurem ents of old passive galaxies,etc.In Chapter 1,we briefly review the research background of cosm ology,and introduce som e cosm ologicalm odels.Then we summarize the progress on cosmology from all kinds of observations in more details.

In Chapter 2,we present the results of our studies on the supernova cosmology.The m ain difficulty with the use of SNe Ia as standard cand les is that onemust optim ize three or four nuisance parameters characterizing SN lum inositiessimultaneously with the parameters ofan expansionmodelof the Universe.Wehave confirmed that one should optim izeallof the parameters by carrying out themethod ofm aximum likelihood estim ation in any situation where the param eters include an unknown intrinsic dispersion.The commonly used m ethod, which estimates the dispersion by requiring the reducedχ2to equal unity,does not take into account all possible variances among the parameters.We carry out such a com parison of the standardΛCDM cosmology and the Rh=ct Universe using the SN Legacy Survey sam ple of 252 SN events,and show that each model fits its individually reduced data very well.Moreover,it isquiteevident that SLSNe Icmay beuseful cosmologicalprobes,perhaps even out to redshiftsmuch greater(z≫2)than those accessible using SNe Ia.However, the currently available sam p le of SNe Ia is still quite small.Our simulations have shown that if SLSNe Ic can be commonly detected in the future,they have the potentialof greatly refining themeasurement of cosmological parameters,particularly the parameter wdeof the dark energy equation of state.

In Chapter 3,we focus on GRB cosmology.We firstly use GRBs as standard cand les in constructing the Hubble diagram at redshifts beyond the current reach of SNe Ia observations.Then wem easure high-z star form ation rate(SFR)using GRBs.We con firm thatthe latest Sw ift sample of GRBs reveals an increasing evolution in the GRB rate relative to SFR at high redshifts.The observed discrepancy between the GRB rate and the SFR may be elim inated by assum ing a cosm ic evolution in metallicity.Assum ing that the SFR and GRB rate are related via an evolvingmetallicity,we find that the GRB data constrain the slope of the high-z SFR to be−2.41.In addition,first stars can only form in structures that are suitably dense,which can be param eterized by them inimum dark matter halomass Mmin.Mminmust p lay an important role in star formation.We can constrain Mmin＜1012.5M⊙at 68%confidence level from the GRB data.

In Chapter 4,we assemble a catalog of 69 strong gravitational lensing system s,and carefully introduce how to constrain cosmological parameters using these im portant data. We find that bothΛCDM and the Rh=ct Universe account for the lens observations quite well,though the precision of these m easurem ents does not appear to be good enough tofavor onem odel over the other.

In Chapters 5 and 6,we use measurements of the galaxy-cluster angular diameter distances and 32 agemeasurements of passively evolving galaxies to test and com pare the standard m odel(ΛCDM)and the Rh=ct Universe,respectively.We show that bothmodels appear to account for these two data very well.However,because of the different number of free parameters in thesemodels,we have to judge the goodness-of-fit of cosmologicalmodels with selection tools,such as the Akaike,Ku llback,and Bayes Inform ation Criteria,favoring Rh=ct overΛCDM with a likelihood of about 70%,75%,and 80%,respectively.

Finally,some open questions and an outlook in the cosmology field are summarized in Chapter 7.

Constrain ing Cosm o logical M odels with D ifferent Observations

WEIJun-jie

(Pu rp le M oun tain O bserva to ry,Chinese A cadem y of Sciences,Nan jing 210008)

10.15940/j.cnki.0001-5245.2016.04.012

†2015-07-01获得博士学位,导师:紫金山天文台吴雪峰研究员、美国亚利桑那大学Fu lvio Melia教授;jjwei@pm o.ac.cn