Stephen Hawking, George Ellis, and Roger Penrose extended the equations for general relativity to include space and time. Not only space, but also time has a beginning - at the Big Bang.
The Beginning of Time
How can Carl Sagan give conclusion that the universe has existed eternally when it is logically invalid?
The concept of infinite past of physical objects is not possible.
An event (like a death of a star) involves finite series of events or time before the event occurred.
Let's just say that before the Big Bang, there were simple forms of Gas and Dust. If these Gas and Dust were always there (eternal), then how come the Big Bang exploded some 14 Billion Years ago? why not 500 Billion years ago?
Simply put, the idea of an infinite past of physical objects is not possible.
Even NASA has acknowledged, that our Universe is about 14 Billion years old. That means to say, the universe, had a beginning.
WMAP- Age of the Universe