Nobelprize Physics 2011 −  Accelerating universe?

Klaas de Boer, Univ. of Bonn

In the 1990 it was discovered that the expansion of the universe was accelerating. This discovery is based on measurements of a special kind of supernovae. The nature of the work is described and the interpretation of the data is explained. How can it be, that the expansion accelerates? How certain are we about it all?

Contents:   Expansion behaviour not understood   Types of supernovae   SNIa as standard candles   Acceleration and Supernovae Ia   Figure acceleration behaviour   Nobel prize 2011   Uncertainties and questions  

The behaviour of the universe is poorly understood

The universe was originally thought to be eternal and unchanging. With a little further thought it is clear this is too simple a view. After all, stars shine and transform matter into light. Also, diffuse gas clouds can condense to dark clouds and in it new stars may form. And stars can explode as supernovae. So clearly, changes take place.

Around 1900, first models for the behaviour of the universe were constructed. Essential contributors to the theory were de Sitter, Einstein, Lemaitre and Friedman. The universe was still considered to be "static". Then the extensive observations by Hubble substantiated earlier reports that the universe is expanding. This led to a rethink of the meaning of the models, to a reconsidering of various aspects of the mathematical equations decribing the model. The essence of how the expansion was found and what the models tell us is given in "The poorly understood behaviour of the universe".

One problem with the study of the universe is that it is not easy to determine distances, in particular when they are thought to be large. The best method is to find and define objects with well known and well understood characteristics. The light they emit extends in all directions and the brightness as we see it is determined by the distance: objects are dimmer proportional to the square of the distance.

Types of Supernovae

Objects that are very bright can be seen even over the far stretches of the universe. Among the brightest objects we know are the supernovae, the stars exploding at the end of their life, releasing gigantic amounts of energy. Over the years one has recognised that such explosions can come from stars with widely different histories. There are essentially three types of supernovae: Type Ia, Type Ib, and Type II.
Type II   Stars born heavy may explode a few million years after their birth and the expulsed material contains the outer shell, still with lots hydrogen; this leaves a clear signature in the spectrum of the light: Supernova Type II. The leftover object is a "pulsar".
Type Ib   Somewheat less heavy stars loose in the course of their life much of the outer hydrogen layers and they explode without the hydrogen spectral signature: this is a Supernova Type Ib.   Also in this case is the leftover object a "pulsar".

Mass exchange between low mass stars may lead to a supernova Type Ia; numbers refer to the actual mass of the star in each stage

Type Ia   Numerous stars are born in pairs. Stars born as a close pair will have several phases of exchange of mass. If conditions are right (and they often are), one of the stars being near its end gets dimmer, BUT may accumulate matter from the companion (see figure). This process may repeat itself (although in each instance some mass may be lost from the system). If the collecting star assembles that much that it surpasses the mass limit of 1.4 times that of the Sun, then it explodes, in fact it deflagrates (supernova) and no stellar remnant is left. The spectrum has no hydrogen signature but other features are special to it: this is Supernova Type Ia (SNIa).

Supernovae Ia as standard candles

Since the SNIa explode always with a total mass of 1.4 that of the Sun they are thought to become equally bright as supenova. Note that before the explosion such stars are inconspicuous, during the explosion they become within a few days very bright and then they fade again, to hardly be discernable after a year or so. Still, for the study of the universe these are the much sought standard candles because such supernovae can be seen even in distant galaxies.

In the early 1990s a system was set up to automatically search other galaxies for the occurance of supenovae. Telescopes were programmed to take snapshot pictures (with an electronic camera) of a large number of galaxies. Each picture was compared with a reference picture in the system's database. If the imaged galaxy was largely the same, nothing happened. But if the recent picture showed a bright spot then "hey bingo", this may be a supernova. Then a larger telescope was trained on the bright spot, a spectrum was recorded and then inspected for the spectral signatures. Could this be a SNIa? If yes, several more spectra were taken that allowed to measure the cosmological redshifting of the spectral features. Also brightness measurements were carried out, in particular to further ascertain the nature of the brightening and dimming (expecting it to be characteristic for SNIa) as well as to accurately measure the maximum brightness of the event.

Interpreting the SNIa measurements

Now comes the tricky part. The maximum brightness is compared with the reference brightness. The difference is a measure for the distance of the SNIa unless other factors, such as obscuration by dust in the Milky Way, had led to some extra dullness. The spectra are used to determine the redshifting of the spectral lines. This is also tricky, since the material expelled from the dying star moves in all directions, so also in ours, which leads to some extra Dopler shift. Anyway, if all is taken care of, then one has a data point for a graph of distance versus the cosmic expansion, much like the diagram Hubble developed in the late 1920s (see Hubble galaxies).

Simple models for the expansion behaviour of the universe. From top to bottom:
Yellow: continuously accelerating expansion. Red: constant expansion. Green: coasting expansion. Blue: expansion and ultimate contraction.
One can tweak the model parameters to get any model leading to a model curve between the blue and the yellow one. Measurements of SNIa lie in the shaded range.

Measuring galaxies and distant supernovae means we measure into the past. The data on redshift and distance are entered in the diagram of the behaviour of the universe, a graph with model behaviours (see also Figure expansion of universe). The figure shown here is an enlargement of that portion of the one referred to, depicting the here-and-now and the past. One then attempts to recognise the model that fits best. This is done by eye or one applies a numerical analysis. In the first case the match can be subjective, in the second case one trusts the software, which may be unjustified, or trusts the outcome which may be unjustified since software cannot "filter" extreme and strange data points. In any case, a best fitting model is the one that matches the data in the best way. Which need not mean that the model is correct or that a different model might be better (or more realistic).

The currently available SNIa data cover approximately the range of R(t)/R0 as shown by the shading in the figure. Thus, over the limited stretch of distance available from the data, the models only marginally differ and the interpretation is not very precise. The claim is that the data deviate from the green curve in an upward manner, as if the green model should transit to a shifted yellow model.

The most favoured model assumes that in the equations the "curvature parameter" k (see Figure expansion of universe) equals zero such that k= 1 − (matter density) − (vacuum density). (The "vacuum density" is another name for the "cosmological constant" that is needed in the models to allow for a gravitating stationary universe.) Note that k=0 is aestetically nice and refers to a "flat" universe (thus with "normal" geometry).

Nobel prize 2011

The Nobel prize 2011 was awarded to the team having discovered and substantiated that further into the past the model for the behaviour of the universe fitting the SNIa data is one that suggests the expansion of the universe was slower in the past, thus that it is now accelerating. They have invested large amounts of time and have discovered a possibly very disturbing aspect of the universe. However, given all the uncertainties, awarding the Nobel prize 2011 for this work has also been called premature.


Uncertainties and questions

There are several problems with the assumptions made in the science of the evolution of the universe, especially with the accelerating expansion.

Are SN Ia standard candles? Newer investigations of how the special stars leading to a SNIa revealed a fundamental problem. These special stars can, as explained, come to an explosion if they accumulate mass from a companion. The explosion occurs when the total mass is 1.4 times that of the Sun. So far that is OK. However, if the star that collects this mass transfer rotates, it can accumulate more mass before exploding; in fact, the centrifugal force permits a larger mass load before the stellar core decides to give in. Thus the explosion would occur when the star has a total mass of more than 1.4 that of the Sun. Then the explosion would shine brighter. And brighter supernovae are easier to detect. Furthermore, the models themselves for the explosion are being refined casting also doubt on the standard candle concept.

Why are models with k=0 preferred? The favoured idea is that the "curvature parameter" k= 1 − (matter density) − (vacuum density) =0. Models with k=0 are nice. The universe has a start, initially expands fast and expansion slows down gently. According to what is called "Occams razor" the most simple model (like k=0) is therefore the favoured one. That assumption need not be correct.

Homogeneity of the universe. The equations describing the structure of the universe are, together with relativity theory of Einstein, mathematically complicated and can be dealt with only if one assumes matter is distributed in a homogenous way in the universe. By and large the universe seems to be homogeneous, i.e., looks much the same in all directions. All other models are mathematically quite more complicated and have thusfar not been explored in much detail. That is understandable because one shies away from this complexity because of the tediousness of the work.

But it has become clear that there are regions in which the density of matter (number of galaxies) differs considerably from the average, in fact is very low, regions called "voids". A theoretician in Christchurch (NZ) has made a "toy universe" consisting of regions with above average and regions with below average density. In this toy universe the expansion is not monotonous, the effect of redshift is not cumulative and the age of the universe can be quite larger than the 13.4 Gyr as derived for the homogeneous case. Already the first attempt to make the models for the universe more realistic shows that the unexplained aspects, like the seeming acceleration of the expansion, need not be in conflict with a non-exotic model.

KSdB, 2017.08.13     orioginal 2011.10.11,   nobel2011.html