Failure of the Steady State Theory I was reading a journal of astronomy and came to the most famous opponent of Big Bang theory:

The Steady State Theory:
The 20th - century theory was proposed by Hoyle,Gold and Bondie. The theory is based on the Perfect Cosmological Principle which states that universe has no change in its appearance and is homogeneous. It is isotropic in nature. When an old star dies,new star replaces it. So everything remains the same . According to the theory, the universe has neither any beginning nor any end. Universe was and will always the same through the whole time.

Then I was surprised when the journal wrote that this theory has no scientific existence now. It is obsolete today. While reading this, I have found nothing wrong. Isn't the universe isotropic? Why did the Steady State theory fail?
 A: The steady state theory fails to model a few observed features of the universe:


*

*the accelerated expansion of the universe

*radio galaxies and quasars that are only observed at high redshifts & not everywhere

*the existence of the microwave background light
A: Here is a good basic summary of the history of the steady-state theory and the observations that caused it to fall out of favor, mainly the second two mentioned by Kyle Kanos. One of these was the observation of intense radio sources that didn't seem evenly distributed throughout the universe, but were only seen at large distances (higher redshifts):

The program at Cambridge was led by Martin Ryle, who in 1974 would
  receive the Nobel Prize in physics for his overall contributions to
  radio astronomy. In 1951 Ryle believed that radio sources were located
  within our galaxy, and hence were of no cosmological interest. But
  over the next few years he became convinced that most of the radio
  sources he was detecting were extragalactic. His observations, then,
  could be used to test cosmological models. Ryle argued that his survey
  of almost 2,000 radio sources, completed in 1955, contradicted
  steady-state theory, because more distant/older sources seemed to be
  distributed differently from nearby ones. But he overstated the
  significance of his initial data. Only after more years of work would
  radio observations argue strongly against steady-state theory.

And the other was the blackbody spectrum of the cosmic microwave background radiation, which is naturally predicted by the Big Bang theory but not by the steady-state theory (Alpher and Herman had predicted it in the 1940s and the idea had been rediscovered by Robert Dicke and Yakov Zel'dovich in the early 1960s, all before its actual discovery in 1965):

In 1963 Arno Penzias and Robert Wilson, studying the sky's microwave
  "noise" for Bell Telephone Laboratories, realized that they had
  detected microwaves coming from all around the sky, a universal
  background radiation. Robert Dicke, a physicist nearby at Princeton
  University, learned of the measurement and in 1965 correctly
  interpreted it as radiation of about 3 degrees Kelvin, left over from
  the big bang. Dicke had not known about Alpher and Herman's
  prediction, and had independently thought of the cosmic background
  radiation. Even before learning of Penzias and Wilson's observation,
  Dicke had set his former student James Peebles to work on calculating
  the nature of this radiation. Only later was Alpher and Herman's
  predition recovered and appreciated.
...
The minority who still preferred steady-state theory were not
  convinced that the big bang had been detected. Couldn't Penzias and
  Wilson's single observation have some other explanation? Hoyle argued
  that the radiation could come from interaction between stellar
  radiation and interstellar needle-shaped grains of iron. Only a number
  of measurements at different frequencies could confirm that the
  radiation had the properties predicted for a remnant of the big bang.
  It was not until the early 1970s that techniques advanced enough to
  make these measurements, bringing abundant confirmation.
For most purposes, however, the debate between the big bang and the
  steady state was over in 1965, with big bang the clear winner.
  Steady-state advocates were reduced to making ad hoc arguments of
  little plausibility, and they were increasingly marginalized.

This page goes into some more technical detail, and mentions another issue brought up in Godparticle's answer, that of the abundance of helium, which can be explained easily by Big Bang nucleosynthesis but not by the steady-state theory: 

The Big Bang was originally proposed in the context of making all the
  elements. But the lack of a stable nucleus with atomic weight A=5
  meant that only isotopes of hydrogen, helium and a trace of lithium
  are produced in Big Bang Nucleosynthesis. In the original Steady
  State proposal, all of the heavy elements were produced in stars by
  burning hydrogen into helium and then combining several helium nuclei
  [alpha particles] into heavier nuclei like carbon (3 alpha particles)
  and oxygen (4 alpha particles). In general the heavy element
  abundances relative to hydrogen are proportional to each other. Some
  stars have very little oxygen and these usually also have very little
  iron, and so on. But helium is definitely an exception to this rule.
  There is a non-zero floor to the helium abundance as the oxygen
  abundance goes to zero. This is shown in the plot at right which shows
  the helium and oxygen abundances relative to hydrogen by number of
  nuclei in the Sun and several ionized hydrogen nebulae [H II regions]
  in our Milky Way [M42 is the Orion nebula, M17 is the Omega nebula],
  in the nearby dwarf galaxies known as the Large and Small Magellanic
  clouds [LMC and SMC], and in other extragalactic H II regions. 

This plot clearly shows that solid line, which allows for the
  primordial helium produced in the Big Bang, is a much better fit than
  the dashed line which is the prediction of the Steady State model with
  no primordial helium. The data for this plot were taken from Figure 1b
  of a recent paper on the element abundances in the Sun. Shortly
  before the discovery of the CMB killed the Steady State model, Hoyle &
  Tayler (1964, Nature, 203, 1008) wrote "The Mystery of the Cosmic
  Helium Abundance" in which they decided that most of the helium in the
  Universe was not produced in stars. Hoyle held open the possibility of
  explosions in supermassive objects instead of a single Big Bang, but
  ordinary stars were ruled out.

The page also mentions that although Hoyle abandoned the original steady-state model, he continued to advocate a "quasi-steady state cosmology" (QSSC) where the universe continually expands and contracts in an oscillatory way, and the author of the page gives some technical arguments as to why this model still can't adequately account for all the observations that are generally taken to support the Big Bang model.
A: The following passage has been extracted from the book Parallel worlds:  

Finally, in Nature magazine in 1965, Hoyle officially conceded defeat,
  citing the microwave background and helium abundance as reasons
  for abandoning his steady state theory.  

A: The main problem for steady state theory now would be  bow can steady state theory produce dark matter and baryonic matter?
That is a big problem for the theory.It would be hard to find a conservation law for this process.
