The early universe

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1. Space, time & radiation all created in Big Bang

   • At first universe was hot glowing ball of radiation & matter, temperature fell as it expanded
     • universe now cold + dark (besides stars)
       • Stars we see at night in the Milky way galaxy - our home galaxy
         • Galaxy is a collection of billions of stars held together by their own gravity
         • Light from furthest galaxies taken billions of years to reach us - powerful telescopes give picture of universe long ago

2. Before galaxies + stars formed, universe was dark patchy cloud of hydrogen & helium

   • Dark age of the Universe

3. Force of gravity pulled matter into galaxies and stars

   • Uncharged atoms don't repel each other but they can attract each other
     • During dark age of universe, force of gravitational attraction was at work without any opposition from repulsive forces
     • Gravity between 2 helium nuclei unable to pull them together as they are both positively charged
       • force of repulsion much greater than force of gravity between them
   • As universe continued to expand, became more patch as denser parts attracted nearby matter
     • Gravity pulled more matter into denser parts, turned them into gigantic clumps which eventually turned into galaxies and stars by force of gravity