{
"cells": [
{
"cell_type": "markdown",
"id": "9e8afe14-79cd-4ef9-8024-45638010e642",
"metadata": {},
"source": [
"# Cosmology "
]
},
{
"cell_type": "markdown",
"id": "f92b3c07-80bb-472e-be8c-cfcff2d305b2",
"metadata": {},
"source": [
"## Section 1: Setup the metric "
]
},
{
"cell_type": "code",
"execution_count": 1,
"id": "27ba461d-dd62-4539-bc64-e0610d03a55c",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
"\n",
"\n",
"\t\t\t\t\t\t\t\t\t\t\t\t"
],
"text/plain": [
"\n",
"\n",
"\n",
"\n",
"\n",
"1 0 0 0\n",
"\n",
" 2\n",
" R[t]\n",
" -(------------)\n",
" D 2\n",
" 1 + - - k r\n",
"0 r 0 0\n",
"\n",
"\n",
"\n",
"\n",
"\n",
" 2 2\n",
"0 0 -(r R[t] ) 0\n",
"\n",
"\n",
"\n",
"\n",
"\n",
" 2 2 2\n",
"0 0 0 -(r R[t] Sin[θ] )\n",
"/Users/jamjang/Desktop/coscom/mathematica/tutorial"
]
},
"execution_count": 9,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"(* Defining coordinate system *)\n",
"coord = {t, r, θ, ϕ};\n",
"(* Set these variable for coordinate, hopefully mathematica will be \\\n",
"smarter when we Simplify *)\n",
"(* Noted that e is an expansions parameter *)\n",
"$Assumptions = t ∈ Reals && r ∈ Reals && θ ∈ Reals && ϕ ∈ Reals && e ∈ Reals;\n",
"(* Defining the metric*)\n",
"metric = DiagonalMatrix[{1, -(R[t]^2/(1 - k r^2 + D/r)), -R[t]^2 r^2, -R[t]^2 (r*Sin[θ])^2}];\n",
"metricsign = -1;\n",
"metric // MatrixForm\n",
"(* Load the package from the folder which you put diffgeo.m *)\n",
"SetDirectory[\"/Users/jamjang/Desktop/coscom/mathematica/tutorial\"]\n",
"<< diffgeo.m"
]
},
{
"cell_type": "markdown",
"id": "ed97ca99-c706-4900-a6d5-b802f5218c10",
"metadata": {},
"source": [
"## Section 2 : The equation of motion, typical Einstein equation + cosmological constant\n",
"(* axion can be straightforwardly added *)"
]
},
{
"cell_type": "code",
"execution_count": 13,
"id": "b9be4a37-b272-4cde-8eba-5b993a2d72ac",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
"Einstein tensor, with both indices down.\n",
"Einstein := (Unprotect[Einstein]; \n",
"\n",
" \n",
"> Einstein = Simplify[RicciTensor - RicciScalar*(metric/2)]; Protect[Einstein];\\\n",
" \n",
"> Einstein)\n",
" \n",
"\n",
"\n",
"Attributes[Einstein]={Protected}"
]
},
"execution_count": 13,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"?Einstein"
]
},
{
"cell_type": "code",
"execution_count": 14,
"id": "d33b78b0-2f41-45a1-86ab-a14255a959e2",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" 3 3 2\n",
" D - k r - r R'[t]\n",
" --------------------\n",
"{ϕ, θ, θ, ϕ} r\n",
"\n",
" 3 3 2\n",
" -D + k r + r R'[t]\n",
" ---------------------\n",
"{θ, ϕ, θ, ϕ} r\n",
"\n",
" 2 3 3 2\n",
" Sin[θ] (-D + k r + r R'[t] )\n",
" -(-------------------------------)\n",
"{θ, ϕ, ϕ, θ} r\n",
"\n",
" 2 3 3 2\n",
" Sin[θ] (-D + k r + r R'[t] )\n",
" -------------------------------\n",
"{ϕ, θ, ϕ, θ} r\n",
"\n",
" 3 3 2\n",
" 1 D + 2 k r + 2 r R'[t]\n",
" -(-) ------------------------\n",
"{r, θ, θ, r} 2 r\n",
"\n",
" 3 3 2\n",
" D + 2 k r + 2 r R'[t]\n",
" ------------------------\n",
"{θ, r, θ, r} 2 r\n",
"\n",
" 3 3 2\n",
" 1 D + 2 k r + 2 r R'[t]\n",
" -(-) ------------------------\n",
"{θ, r, r, θ} 2 2 3\n",
"{ϕ, r, r, ϕ} r (D + r - k r )\n",
"\n",
" 3 3 2\n",
" D + 2 k r + 2 r R'[t]\n",
" ------------------------\n",
"{r, θ, r, θ} 2 3\n",
"{r, ϕ, r, ϕ} 2 r (D + r - k r )\n",
"\n",
" 2 3 3 2\n",
" 1 Sin[θ] (D + 2 k r + 2 r R'[t] )\n",
" -(-) ----------------------------------\n",
"{r, ϕ, ϕ, r} 2 r\n",
"\n",
" 2 3 3 2\n",
" Sin[θ] (D + 2 k r + 2 r R'[t] )\n",
" ----------------------------------\n",
"{ϕ, r, ϕ, r} 2 r\n",
"\n",
"{t, r, t, r} R''[t]\n",
"{t, θ, t, θ} -(------)\n",
"{t, ϕ, t, ϕ} R[t]\n",
"\n",
"{r, t, t, r} R''[t]\n",
"{θ, t, t, θ} ------\n",
"{ϕ, t, t, ϕ} R[t]\n",
"\n",
" 2\n",
"{t, θ, θ, t} -(r R[t] R''[t])\n",
"\n",
" 2\n",
"{θ, t, θ, t} r R[t] R''[t]\n",
"\n",
" r R[t] R''[t]\n",
" -(-------------)\n",
" 3\n",
"{t, r, r, t} D + r - k r\n",
"\n",
" r R[t] R''[t]\n",
" -------------\n",
" 3\n",
"{r, t, r, t} D + r - k r\n",
"\n",
" 2 2\n",
"{t, ϕ, ϕ, t} -(r R[t] Sin[θ] R''[t])\n",
"\n",
" 2 2\n",
"{ϕ, t, ϕ, t} r R[t] Sin[θ] R''[t]"
]
},
"execution_count": 14,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"display[Riemann]"
]
},
{
"cell_type": "code",
"execution_count": 15,
"id": "cf6289cc-0c75-4d34-b54d-bfdb34740275",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" 2\n",
"-6 (k + R'[t] + R[t] R''[t])\n",
"-----------------------------\n",
" 2\n",
" R[t]"
]
},
"execution_count": 15,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"RicciScalar"
]
},
{
"cell_type": "code",
"execution_count": 16,
"id": "bee1958b-8294-4f07-9ba8-06df58b0798e",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" 2\n",
" 3 (k + R'[t] )\n",
" --------------\n",
" 2\n",
"{t, t} R[t]\n",
"\n",
" 3 3 2 3\n",
" -D + k r + r R'[t] + 2 r R[t] R''[t]\n",
" -(----------------------------------------)\n",
" 2 3\n",
"{r, r} r (D + r - k r )\n",
"\n",
" 3 3 2 3\n",
" 1 D + 2 k r + 2 r R'[t] + 4 r R[t] R''[t]\n",
" -(-) -------------------------------------------\n",
"{θ, θ} 2 r\n",
"\n",
" 2 3 3 2 3\n",
" 1 Sin[θ] (D + 2 k r + 2 r R'[t] + 4 r R[t] R''[t])\n",
" -(-) -----------------------------------------------------\n",
"{ϕ, ϕ} 2 r"
]
},
"execution_count": 16,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"display[Einstein]"
]
},
{
"cell_type": "code",
"execution_count": 17,
"id": "e310ce84-735b-47f8-9b32-859cf0e68120",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" -3 R''[t]\n",
" ---------\n",
"{t, t} R[t]\n",
"\n",
" 1 D 2 2 2 2\n",
" -(-) - + 2 k r + 2 r R'[t] + r R[t] R''[t]\n",
"{θ, θ} 2 r\n",
"\n",
" 3 3 2 3\n",
" D + 2 k r + 2 r R'[t] + r R[t] R''[t]\n",
" -----------------------------------------\n",
" 2 3\n",
"{r, r} r (D + r - k r )\n",
"\n",
" 2 3 3 2 3\n",
" Sin[θ] (-D + 4 k r + 4 r R'[t] + 2 r R[t] R''[t])\n",
" ------------------------------------------------------\n",
"{ϕ, ϕ} 2 r"
]
},
"execution_count": 17,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"display[RicciTensor]"
]
},
{
"cell_type": "code",
"execution_count": 18,
"id": "df17587c-7c66-49e7-af0e-ebe594f29ef8",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
"lower[ tensor, {index1,index2,...} (optional) ] lowers index1, index2, etc. using the\\\n",
" \n",
"> metric; if the list of indices is omitted then all indices are lowered (so they\\\n",
" \n",
"> should all be up in the original tensor).\n",
"lower[(tensor_)?ArrayQ] := lower[tensor, Range[ArrayDepth[tensor]]]\n",
" \n",
"lower[(tensor_)?ArrayQ, indic_List] := \n",
"\n",
" \n",
"> Simplify[Fold[Transpose[Inner[Times, #1, metric, Plus, #2], \n",
"\n",
" \n",
"> Join[Range[#2 - 1], Range[#2 + 1, ArrayDepth[tensor]], {#2}]] & , tensor,\\\n",
" \n",
"> indic]]\n",
" \n",
"\n",
"\n",
"Attributes[lower]={Protected}"
]
},
"execution_count": 18,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"?lower"
]
},
{
"cell_type": "code",
"execution_count": 15,
"id": "552288bb-387f-41d5-a089-2972e22e12cf",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" 2\n",
" r u1 R[t] 2 2 2 2 2\n",
"{u0, -(------------), -(r u2 R[t] ), -(r u3 R[t] Sin[θ] )}\n",
" 3\n",
" D + r - k r"
]
},
"execution_count": 16,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"u = {u0, u1, u2, u3};\n",
"ul = lower[u] // FullSimplify\n",
"(*{ud0,ud1,ud2,ud3}=ul;*)"
]
},
{
"cell_type": "markdown",
"id": "a31d545c-6b37-4051-8e97-e1ae4a6c78e1",
"metadata": {},
"source": [
"# $T_{\\mu\\nu} = (\\rho+P)u_{\\mu}u_{\\nu}-g_{\\mu\\nu}P$"
]
},
{
"cell_type": "code",
"execution_count": 18,
"id": "9b14009f-743d-4976-ba00-8a67dc6db874",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" 2 2\n",
"{θ, θ} r P[t] R[t]\n",
"\n",
" 2\n",
" P[t] R[t]\n",
" ------------\n",
" D 2\n",
" 1 + - - k r\n",
"{r, r} r\n",
"\n",
" 2 2 2\n",
"{ϕ, ϕ} r P[t] R[t] Sin[θ]\n",
"\n",
"{t, t} ρ[t]"
]
},
"execution_count": 20,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"usub = {u0 -> 1, u1 -> 0, u2 -> 0, u3 -> 0};(*Comoving observer*) \n",
"Tmunu = (ρ[t] + P[t]) TensorProduct[ul, ul] - (P[t])*metric /. usub;\n",
"display[Tmunu] "
]
},
{
"cell_type": "markdown",
"id": "aaaa69ac-1aca-46d4-83c7-22ca53285b3c",
"metadata": {},
"source": [
"# $G_{\\mu\\nu} = 8\\pi T_{\\mu\\nu}$"
]
},
{
"cell_type": "code",
"execution_count": 25,
"id": "93c887e2-175b-4f08-83f2-b9ef9200c1b6",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
"\n",
"\n",
"\t\t\t\t\t\t\t\t\t\t\t\t"
],
"text/plain": [
" 2\n",
"3 (k + R'[t] )\n",
"-------------- == 8 Pi ρ[t]\n",
" 2\n",
" R[t]\n",
" 2 2\n",
" k 8 Pi P[t] R[t] R'[t] 2 R[t] R''[t]\n",
"--------- + --------------- + --------- + ------------- == 0\n",
" 2 2 2 2\n",
"-1 + k r -1 + k r -1 + k r -1 + k r"
]
},
"execution_count": 25,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"Einstein[[1, 1]] == 8 π Tmunu[[1, 1]] \n",
"(Einstein[[2, 2]] == 8 π Tmunu[[2, 2]] /. D -> 0) // Simplify // Expand"
]
},
{
"cell_type": "code",
"execution_count": 27,
"id": "7d10c79e-f220-424c-9012-66c0a574c258",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" 2\n",
" k R'[t]\n",
"---- + 8 Pi P[t] R[t] + ------ + 2 R''[t] == 0\n",
"R[t] R[t]"
]
},
"execution_count": 27,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"(-1 + k r^2)/R[t]^2 (k/(-1 + k r^2) + (8 π P[t] R[t]^2)/(-1 + k r^2) + Derivative[1][R][t]^2/(-1 + k r^2) + (2 R[t] (R'')[t])/(-1 + k r^2)) == 0 // Simplify // Expand"
]
},
{
"cell_type": "code",
"execution_count": 28,
"id": "df69b930-9475-43f7-a363-ca9978983f9a",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
"{r, r}\n",
"{θ, θ}\n",
"{ϕ, ϕ} -P[t]\n",
"\n",
"{t, t} ρ[t]"
]
},
"execution_count": 29,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"Tdu1 = raise[Tmunu, {2}];\n",
"display[Tdu1]"
]
},
{
"cell_type": "markdown",
"id": "6b67bd23-14ac-40cf-915a-aace7840df6f",
"metadata": {},
"source": [
"# $EOM = R_{\\mu\\nu} - 8\\pi (T_{\\mu\\nu} - g_{\\mu\\nu}T)$"
]
},
{
"cell_type": "code",
"execution_count": 30,
"id": "bbde89f2-109c-471b-ba3e-6bb951bdaa5d",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" 3 R''[t]\n",
"{{-12 Pi P[t] - 4 Pi ρ[t] - --------, 0, 0, 0}, \n",
" R[t]\n",
" \n",
"> {0, \n",
" \n",
" 3 3 2 3 2 3 2 3\n",
" D + 2 k r + 4 Pi r P[t] R[t] - 4 Pi r R[t] ρ[t] + 2 r R'[t] + r R[t] R''[t]\n",
"> -----------------------------------------------------------------------------------\n",
" 2 3\n",
" r (D + r - k r )\n",
" \n",
" 1 D 2 2 2 2 2\n",
"> , 0, 0}, {0, 0, -(-) - + 2 k r + 4 Pi r P[t] R[t] - 4 Pi r R[t] ρ[t] + \n",
" 2 r\n",
" \n",
" 2 2 2\n",
"> 2 r R'[t] + r R[t] R''[t], 0}, \n",
" \n",
" 2 3 3 2 3 2\n",
"> {0, 0, 0, (Sin[θ] (-D + 4 k r + 8 Pi r P[t] R[t] - 8 Pi r R[t] ρ[t] + \n",
" \n",
" 3 2 3\n",
"> 4 r R'[t] + 2 r R[t] R''[t])) / (2 r)}}"
]
},
"execution_count": 30,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"EOM = RicciTensor - 8 π (Tmunu - metric (Tdu1[[1, 1]] + Tdu1[[2, 2]] + Tdu1[[3, 3]] + Tdu1[[4, 4]])/2) // Simplify"
]
},
{
"cell_type": "code",
"execution_count": 31,
"id": "adab908b-59dd-4877-ab16-ab410d9331a2",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
"\n",
"\n",
"{t, t} 3 R''[t]\n",
" -12 Pi P[t] - 4 Pi ρ[t] - --------\n",
" R[t]\n",
"\n",
"\n",
"\n",
"{θ, θ} 1 D 2 2 2 2 2 2 2\n",
" -(-) - + 2 k r + 4 Pi r P[t] R[t] - 4 Pi r R[t] ρ[t] + 2 r R'[t] + \n",
" 2 r\n",
" \n",
" 2\n",
" > r R[t] R''[t]\n",
"\n",
" 3 3 2 3 2 3 2\n",
"{r, r} (D + 2 k r + 4 Pi r P[t] R[t] - 4 Pi r R[t] ρ[t] + 2 r R'[t] + \n",
" \n",
" 3 2 3\n",
"> r R[t] R''[t]) / (r (D + r - k r ))\n",
"\n",
"\n",
"{ϕ, ϕ} 2 3 3 2 3 2 3 2\n",
" (Sin[θ] (-D + 4 k r + 8 Pi r P[t] R[t] - 8 Pi r R[t] ρ[t] + 4 r R'[t] + \n",
" \n",
" 3\n",
" > 2 r R[t] R''[t])) / (2 r)"
]
},
"execution_count": 31,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"display[EOM]"
]
},
{
"cell_type": "code",
"execution_count": 32,
"id": "1c976b72-c7a0-4c2b-ad8e-ecf1c9eac10a",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" 3 D 2 2\n",
"3 k - ---- - 8 Pi R[t] ρ[t] + 3 R'[t]\n",
" 3\n",
" 4 r"
]
},
"execution_count": 32,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"(EOM[[3, 3]] /. {R''[t] -> -((4*π)/3) R[t] *(ρ[t] + 3 P[t])}) 3/(2 r^2) // Simplify"
]
},
{
"cell_type": "code",
"execution_count": 33,
"id": "4c8417a6-1501-410d-b311-ce9f363d8376",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" 2\n",
" D 8 Pi R[t] ρ[t] 2\n",
"---- + --------------- == k + R'[t]\n",
" 3 3\n",
"4 r"
]
},
"execution_count": 34,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"(*Only way [[1,1]] consistent with [[3,3]] is D=0*)\n",
"D/(4 r^3) + 8/3 π R[t]^2 ρ[t] == (k + Derivative[1][R][t]^2)"
]
},
{
"cell_type": "code",
"execution_count": 35,
"id": "ce7fdc84-e8c8-4436-b95d-a4e14592fd93",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
],
"text/plain": [
" 2\n",
"8 Pi R[t] ρ[t] 2\n",
"--------------- == k + R'[t]\n",
" 3"
]
},
"execution_count": 36,
"metadata": {
"text/html": [],
"text/plain": []
},
"output_type": "execute_result"
}
],
"source": [
"(*ALSO Friedmann Eqn for D=0*)\n",
"8/3 π R[t]^2 ρ[t] == (k + Derivative[1][R][t]^2)"
]
},
{
"cell_type": "code",
"execution_count": 37,
"id": "609a707d-a00f-4df0-b944-c85b2fafe93d",
"metadata": {},
"outputs": [
{
"data": {
"text/html": [
""
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"text/plain": [
"covariant[ tensor, indexpositions (optional) ]: covariant derivative of a tensor; the\\\n",
" \n",
"> derivative index is the first index of the resulting tensor; the indexpositions\\\n",
" \n",
"> argument is a list of the form {up,down,down}; if it is omitted, then all indices\\\n",
" \n",
"> are assumed to be down. (The index position none is also allowed, indicating that\\\n",
" \n",
"> the corresponding index should be ignored, i.e. no connection should be used.)\n",
"covariant[(scalar_)?scalarQ] := partial[scalar]\n",
" \n",
"covariant[(tensor_)?FullTensorQ] := covariant[tensor, ConstantArray[down, rank[tensor]]]\n",
" \n",
"covariant[tensor_, indexp:{(up | down | none)...}] := \n",
"\n",
" \n",
"> Simplify[With[{trank = Length[indexp], Christoffelt = Transpose[Christoffel, \n",
"\n",
" \n",
"> {3, 2, 1}]}, partial[tensor] + Sum[Which[indexp[[index]] === down, \n",
"\n",
" \n",
"> -Transpose[Inner[Times, tensor, Christoffel, Plus, index], \n",
"\n",
" \n",
"> Join[Range[2, index], Range[index + 2, trank + 1], {1, index + 1}]], \n",
"\n",
" \n",
"> indexp[[index]] === up, Transpose[Inner[Times, tensor, Christoffelt, Plus,\\\n",
" \n",
"> index], \n",
" Join[Range[2, index], Range[index + 2, trank + 1], {1, index +\\\n",
" \n",
"> 1}]], \n",
" indexp[[index]] === none, 0], {index, trank}]]]\n",
" \n",
"\n",
"\n",
"Attributes[covariant]={Protected}"
]
},
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"contract[ tensor, {index1,index2}, {index3,index4}, . . . (optional) ]: contraction of\\\n",
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" \n",
"> indices to be contracted are assumed to be down). If no index pair is given, it\\\n",
" \n",
"> returns the contraction of the first two indices.\n",
"contract[(tensor_)?ArrayQ] := tr[inverse . tensor]\n",
" \n",
"contract[(tensor_)?ArrayQ, indic:{_, _}..] := Nest[tr[inverse . #1] & , \n",
"\n",
" \n",
"> transpose[tensor, Join[indic]], Length[{indic}]]\n",
" \n",
"\n",
"\n",
"Attributes[contract]={Protected}"
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"{--------------------- + ρ'[t], 0, 0, 0}\n",
" R[t]\n",
" 3 (P[t] + ρ[t]) R'[t]\n",
"{--------------------- + ρ'[t], 0, 0, 0}\n",
" R[t]"
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"execution_count": 39,
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"source": [
"contract[covariant[Tmunu], {1, 2}]\n",
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{
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"-3 P[t] + ρ[t]"
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{
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"--------------------- + ρ'[t]\n",
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"execution_count": 43,
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"source": [
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"CON[[1]]"
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"text/plain": [
" R0 3 (1 + w) 2\n",
"8 Pi ρ0 (----) R[t]\n",
" R[t] 2\n",
"----------------------------- == k + R'[t]\n",
" 3\n",
" (3 (1 + w))/2 2/(3 (1 + w))\n",
" ((1 + w) (2 Sqrt[6 Pi] R0 t Sqrt[ρ0] + 3 C[1]))\n",
"{{R[t] -> -------------------------------------------------------------------------}, \n",
" 1/(3 + 3 w)\n",
" 4\n",
" \n",
"> {R[t] -> \n",
" \n",
" (3 (1 + w))/2 2/(3 (1 + w))\n",
" (-((1 + w) (2 Sqrt[6 Pi] R0 t Sqrt[ρ0] - 3 C[1])))\n",
"> ----------------------------------------------------------------------------}}\n",
" 1/(3 + 3 w)\n",
" 4"
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},
"execution_count": 44,
"metadata": {
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"source": [
"(8/3 π R[t]^2 ρ[t] == (k + Derivative[1][R][t]^2) /. {P[t] -> w ρ[t]} /. {ρ[t] -> ρ0 (R0/R[t])^(3 (1 + w))})\n",
"DSolve[8/3 π ρ0 (R0)^(3 (1 + w)) R[t]^(-1 - 3 w) == Derivative[1][R][t]^2, R[t], t] // Simplify "
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