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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 33: Einstein's Universe

Readings: none

Key Ideas

Cosmological Principle:
The Universe is Homogeneous and Isotropic on Large Scales.
No special places or directions.

General Relativity predicts an expanding universe.

Einstein's Greatest Mistake (?)


Cosmology is the study of the entire Universe:

The Universe in 1917

Einstein explored the cosmological implications of General Relativity.

Observational State in 1917:

The Cosmological Principle

"The Universe is Homogeneous and Isotropic on the Largest Scales."

Critical assumption underlying Cosmology.



Largest Scales:


When viewed on the largest scales:

Does not apply locally:


When viewed on the largest scales:

Does not apply locally:

The Dynamic Universe

Einstein applied the Cosmological Principle to General Relativity and got a surprise:

Astronomers assured him that no such general motion was observed.

The Cosmological Constant

To make the Universe static, he was required to add a new term in his equations:

The Cosmological Constant, L (Lambda):

At the time there was no physical reason to introduce a Cosmological Constant, it was motivated by a presumption that the Universe must be static.

Cosmic Expansion

1914-1922: Vesto Slipher, Lowell Observatory

Measured the radial velocities of the brightest "spiral nebulae".

Results: 21 out of 25 spirals showed a systematic redshift.

Einstein's Greatest Blunder...

By introducing an ad hoc Cosmological Constant, Einstein missed being able to predict the expansion of the Universe.


Edwin Hubble firmly established the facts of general cosmic expansion observationally in 1929.

State of the Art

Einstein's guess about the homogeneity and isotropy of the Universe was brilliant and far ahead of the scanty empirical data of his time.

Modern observations bear out large-scale homogeneity & isotropy on average:

Modern Cosmological Constant

In current cosmological theory, Lambda reappears in a somewhat altered form as the "vacuum energy" of space.


We'll discuss this surprising new result in a future lecture.
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Updated: 2006 February 19
Copyright Richard W. Pogge, All Rights Reserved.