Samatha meditation feels a space at the top of the head? When  doing walking felt a space  at the  top  of the  head. After done  some meditation  the space  began to expand .What is this?And there is a kind  of a heart beat like  inside the space.
It's just a sign that your mind is starting to enter access concentration.  It's really quite common.  People also report sensations like their head swelling, body twisting, face disappearing, hands inflating, etc., etc., etc.  How these feelings manifest is as unique as the person experiencing them.  They're kind of like your own personal brand of samadhi entry.  The sensations themselves are also not particularly important; you don't need to pay them any mind.  Eventually, as your concentration deepens and you move closer to jhana, your meditation will start to take on a character more reminiscent of those described in the sutras.