Universe in age crisis!
How old is the Universe? How long ago did the Big Bang take place? It may seem like a daunting task to date the birth of something as vast and all-encompassing as the Universe. But according to Edwin Hubble, the indisputable father of modern astronomy, it is not.
In 1929, Hubble observed that light and radiation from distant galaxies are red-shifted. A red shift is drift of the wavelength toward longer values if sources of radiation are receding from the observer. The red shift together with cosmic microwave background radiation (CMBR) cold remnants of the hot and highly energetic radiation that emanated after the Big Bang and red-shifted to low-energy, long wavelength microwaves are proof that the Universe is expanding and its contents are moving away from us. In fact, red shift and CMBR are the living storytellers of the Universe. They tell us that before birth, the Universe was a mathematical singularity with all the mass concentrated into an effectively zero volume. The Universe, therefore, has a finite age.
Hubble also found that there is a direct correlation between the distance of a galaxy and its speed of recession. Galaxies nearer to us are moving away slowly, whereas more distant galaxies are rushing away from us rapidly. If their distance from us and speed of recession can be measured, we can use them as clocks to determine the age of the Universe.
Hubble discovered that the ratio of the speed of recession to the distance of a galaxy is constant, known as the Hubble constant. If the constant is 100 km/second/Megaparsec, it implies that for every Megaparsec (19 million trillion miles) of distance, the velocity increases by 100 km per second. Thus the constant gives the current expansion rate of the Universe. The constancy of the ratio ensures that Hubble constant will be same for any pair of galaxies. The reciprocal of the constant has the dimension of time and is called "Hubble time." It is a measure of the age of the Universe.
The Hubble constant is determined from the best fits to the large body of data on galactic speeds and distances gathered by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft. The data is obtained by analyzing the warmer and cooler spots of the Big Bang's CMBR across the sky. From the fits, the expansion rate is found to be 71 km/second/Megaparsec with accuracy within ± 1%. To convert Hubble's unconventional unit to years, multiply Hubble time by 976 billion.
According to the Standard Cosmological Model (Big Bang), the age of the Universe depends on its shape. (See Is the Universe closed or open? TDS June 26, 2012.) The age of an "open," low-matter density Universe, expanding forever at a constant rate, is simply the Hubble time 13.7 billion years. The age of a "flat" Universe, barely managing to continue expanding forever is two-third of the Hubble time 9.2 billion years. Because of its higher density, a "closed" Universe decelerates rapidly. Consequently, it has been expanding for a shorter time, making it younger than a flat Universe. Its age is half of Hubble time 6.85 billion years.
Astronomers are uncomfortable with these estimates of the age, particularly for the flat and closed Universe, because they are less than the age of some of the oldest stars. Clearly, the Universe cannot be younger than the oldest objects it contains. This is equivalent to saying that the child is older than the biological mother.
Apparently the Universe is suffering from an age crisis. The crisis can be resolved once the actual shape of the Universe is pinned down. That will require an accurate computation of the matter density with the inclusion of all the matter in the Universe visible, invisible, "dark," etc. Meanwhile, there is no reason to doubt the value of Hubble constant as it is deduced from the most reliable source, CMBR.
Finally, Hubble constant should decrease with time. Otherwise, the Universe will not age; it will forever remain stuck at one of the ages mentioned above. In that sense, it is really not a constant but a parameter.
"Age is an issue of mind over matter. If you don't mind, it doesn't matter." Mark Twain.